I want to make a few things clear:

1. No, I don't work for CD Projekt or any subsidiary.
   
2. No, I don't work for Steam either.
   
3. The Guide Image was taken from this reddit post - All credit goes there.
   
4. I'm doing this for the sake of everyone who is in the same situation I once was.

I appreciate if this helped you in any way to rate, like or award ( Me or the Guide ), if it did not, place a comment with the actual problem and steps followed. If you manage to resolve your problem in any way that it is not described here, post it too. I'll add it and commend you for it.

Most of the fixes are described in PCGamingWiki[www.pcgamingwiki.com] (PCGW from now on) - So, much appreciated for the users there.

Sometimes Steam Fails for many reasons ( Faulty System, Faulty HDD / SSHD / SSD, Missing Programs or Programs interfering with it, bad connection, bad or outdated drivers... etc ). A compendium of fixes can be found here. On top of that, patches over patches can make our System bulky and in some degree faulty - Which is nothing else that a Low-level program, so yeah, programs are affected by patches too ( That's why a Clean Installation of our Operating System is important and should be done once in a while - A year / a year and a half is a good start if we make many changes on it. I personally do it every major update ).

But we would never need to do a clean OS install for just ONE PROGRAM, there are commands like SFC that check the system for us and programs like Steam that download themselves by having their executable alone. SFC ( Meaning "System File Checker" ) should ALWAYS be used with dism checking commands ( More of it in my section ).

Many times people claim that their system is OK, but in reality they are not - Being there, done that - and that's what is making their programs not work properly.
Note: Not every problem could be fixed by checking the system, as written before, Faulty HDD / SSHD / SSD sometimes needs for a new component ( But I'm not going to cover it here ).

Defrag and Chkdsk could also help, not only for quicker access, but better access - Quicker = Less time, Better = Better readability. Chkdsk fix storage problems, while defrag helps with its order.

---

TL;DR
• Systems become faulty overtime from updates or installing / uninstalling programs - Not everytime, but most of it.
  
• SFC is a MUST when checking problems with a program that should work.
  
• Defrag and Chkdsk helps in the process of checking the system - Also improving SFC

Using programs like Driver Booster is recommended for older PCs, where drivers are not available for our OS.

Instead, your should use programs like Aida64 Extreme[www.aida64.com] and search for your components names ( If you don't know them ). Also, while searching for your drivers, use always official sites. For example, if you have an Asus PRIME Z370-A motherboard, go search for it in Asus website or using Google
Another common mistake is to download and update your drivers without further cleanup. That's why installations like the one on Nvidia comes with a clean install option ( It's not 100% clean, but it's better than nothing ), this will only revert all options in Nvidia Control Panel to their default values.

Note : Below information is NOT A MUST but for a better performances and more error free, you should def do it.
I do recommend using DDU ( Display Driver Uninstaller )[www.wagnardsoft.com] for full graphics driver cleanup. It's a powerful tool, so wait for this guide to be complete or ask someone who has already use it before.

For the system we will use SFC in it's full form.

1. Open task manager ( Control + Shift + Escape )
   
2. Click on "File" -> "Run New Task" -> "cmd" and mark the option "Create this task with administrative privileges.".
   
3. And run this commands - One at the time - :
   
   Dism /Online /Cleanup-Image /CheckHealth Dism /Online /Cleanup-Image /ScanHealth DISM /Online /Cleanup-Image /RestoreHealth SFC /scannow

For the storage, we will check both the system drive and the one that the game is installed.

1. Open task manager ( Control + Shift + Escape )
   
2. Click on "File" -> "Run New Task" -> "cmd" and mark the option "Create this task with administrative privileges.".
   
3. And run this command:
   
   CHKDSK <DriveLetter>: /F /V /R /X /B /scan /perf /scan /perf was Introduced in Windows 8, If I recall correctly Example: CHKDSK C: /F /V /R /X /B /scan /perf
   Note that there's a colon ( : ) after the Drive letter.

Since downloads can get corrupted or file expansions could not work properly, verifying game cache is a important process in all this. A detailed explanation can be found here.

引用自 You：

But i don't want to... I just want to play plain vanilla experience.

Not all mods are for crazy experiences, some just add better compatibility to the game or improve some aspects of it. That's where mods like W1 Vanilla remaster[www.nexusmods.com] and Project Mersey - The Witcher Fix Pack[www.nexusmods.com] excel at ( They probably are not the best ones out there, but are the once I used and prove to work - If you have anything better, write it down on the comments ).

A few mods are posted in PCGW[www.pcgamingwiki.com], give them a try.

From PCGW[www.pcgamingwiki.com]:

• Crash at launch[www.pcgamingwiki.com]
  On AMD: DirectX to OpenGL wrapper
  1. Download wrapper[downloads.fdossena.com]
     
  2. Install by copying d3d9.dll, wined3d.dll and libwine.dll to <path-to-game>\System
  
  
  Opt out from Steam Beta client:
  1. Open Steam settings and select the "Account" tab.
     
  2. In "Beta participation" click "Change..." and select "NONE".
     
  3. Steam should ask you to restart the client and download the latest stable release of the client.
  
  
  Alternative - From Steam Support
  • Navigate to your Steam installation
    
  • Right-click on steam.exe (this file is listed as an application and features the black and white Steam logo) and select Create Shortcut
    
  • Right-click on the new shortcut and go to Properties
    
  • Select the Shortcut tab
    
  • In the Target: field add the launch option "-clearbeta" after the steam.exe location
    
  • Click Apply and then OK to close the Properties window
  
  
  Enable windows compatibility mode for "witcher.exe"
  1. Open game folder "The Witcher Enhanced Edition\System".
     
  2. Open witcher.exe properties by right clicking it and then clicking properties.
     
  3. Click compatibility tab and enable compatibility mode.
     
  4. From drop down list select Windows 7 and apply the settings.
  
  
  
• Minimum system requirements not met[www.pcgamingwiki.com]
  Disable the system requirements check
  This fix is known to cause other issues. You might be unable to change the game's resolution above a certain limit, and the texture quality and antialiasing settings might be locked to the lowest settings. To avoid this, try the optional step or the registry edit solution below.
  
  1. Create a shortcut to witcher.exe (the game itself, NOT the game's launcher).
     
  2. Right-click the shortcut and choose Properties.
     
  3. Add -dontforceminreqs to the end of the Target field.
  
  
  Use replacement TexSettings.d2a
  1. Download TexSettings.d2a[www.nexusmods.com]
     
  2. Move the file to <path-to-game>\Data\Override
  Note : The Override folder does not exist by default, create the folder ( if not already created ) and then move the file to that folder.
  
  
  
• No mouse in the main menu[www.pcgamingwiki.com]
  Press Shift
  In some cases (when the intro cinematic is skipped or if opening the Steam overlay) the mouse cursor does not function on the main menu. This can be fixed by holding the key that releases the mouse in Over the Shoulder view(Shift by default).
  
  
  
• Frequent game crashes[www.pcgamingwiki.com]
  Allocate more virtual memory to the program
  1. Download NTCore's 4GB Patch[ntcore.com]
     
  2. Run the program, apply the patch to the witcher.exe located in <path-to-game>\System\witcher.exe
  Note : Requires Admin Rights.
  
  
  Set the game to run in windowed mode
  Use a program such as Borderless Gaming[www.pcgamingwiki.com] to force the game to run in a borderless window.
  
  
  Disable Steam overlay
  Disabling Steam overlay[www.pcgamingwiki.com] fixes the crashing.
  
  
  
• Pre-rendered cutscenes don't display[www.pcgamingwiki.com]
  When audio will still play, but the video will freeze to the last displayed frame.
  
  Lower the lighting setting
  1. Go to Advanced Settings
     
  2. Set Lighting to low. Cutscenes should now display correctly
  You can revert the lighting setting afterwards. However, the setting will need to be altered before each pre-rendered cutscene.
  
  
  
• In-game cut-scenes appear pixelated[www.pcgamingwiki.com]
  Use any 16:9 resolution
  1. Go to Options, Video Options.
     
  2. Choose a 16:9 resolution (1280x720, 1366x768, 1600x900, 1920x1080, 2560x1440, 3840x2160 or 4096x2160, etc.)
  
  
  
• Lights visible through walls[www.pcgamingwiki.com]
  On some systems with SLI enabled, light sources are sometimes visible through walls.
  
  Change SLI rendering mode
  1. In the Nvidia control panel go to "Manage 3D settings."
     
  2. Hit the tab "Program settings" and add <path-to-game>\system\witcher.exe
     
  3. Scroll down through the settings to the one called "Multi-GPU rendering mode" and change it to Force Alternate frame rendering 2.
     
  4. Click Apply.
  
  
  
• Poor performance on AMD[www.pcgamingwiki.com]
  There is an issue where the game caps at 20fps
  None, at some point it might go higher after gameplay.

Windows Defender Ransomware defence is to blame. Just disable it or add an exception for the app and everything is good to go. It also happens with Adobe Photoshop, After Effects and PES2021. (Those that I know of ).

1. Start Menu -> Options -> Update & Security - Left Side -> Windows Security -> Open Windows Security - Or just go to your Systray - Show hidden Icons -> Double click on the Windows Defender Shield
   
2. Virus & Threat Protection - Scroll down to Ransomware Protection -> Manage Ransomware Protection -> Add an exception or disable it ( Better to add an exception )

As a staff member pointed me out a purge of steam files is also beneficial for the cause.

Purging Steam is easy, just go to your Steam Install directory, and delete everything except for this two folders config and steamapps and this files ssfn#, steam.exe and uninstall.exe. If you can see hidden files, there will be another ssfn file ( # equals numbers ).

I've made a batch file which check everything from here, while also purging steam - If you are lazy to do it yourself - You can find it here[github.com] - Full automatic batch could fail due to system problem. If you are going to try it, copy the batch in an empty folder, since a few people in reddit told me that it cleaned up their Download folder ( Still checking how to fix it, but can't reproduce the failure to begin with ).

By verifying the game cache, it should install the required programs by itself but, if by any chance it did not, install DirectX 9[www.microsoft.com] and Microsft Visual C++ 2005 SP1 Redistributable Package (x86)[www.microsoft.com] ( Those files can also be found in <path-to-game>\directx\DXSETUP.exe and <path-to-game>\vcredist\vcredist_x86.exe ).

This is a simple Workaround that I did know of but gonna give credit to Icedfate - Steam Discussion.