The game uses the TAGES copy protection system, which has has been found to conflict with disc image drive emulators and react similarly to presence of SCSI and SATA drivers in the system, leading to a number of occasions where the copy protection system has prevented users from running legitimately purchased copies of the game.
These problems can be usually avoided by uninstalling the TAGES driver with the official installation program and then starting the game (which will automatically install the appropriate driver version).
How to remove TAGES ? ( while using patch 1.5 )
Given that patch 1.5 removes the copy protection scheme known as TAGES from the game executable, here is how to remove TAGES from your system.
First of all you must establish if TAGES is needed by any other games that you have installed, unfortunately we can not do that for you, and no list of TAGES protected games was found by me (if you have such a list do share it so I can update this). Presuming that you do not have any other game that needs TAGES, you can carry on, get the latest TAGES executable, for your Windows 32bit or Windows 64bit, from here, and run it.
You will get one of two possible dialogue windows:
If you are asked if you want to install TAGES, it means that TAGES is not installed on your system, so you will just press NO and that is it (IIRC, the original game updated with patch 1.4 works even without the TAGES system files).
If you are asked if you want to uninstall TAGES, it means you have TAGES installed on your system, so you will just press yes and that is it. You might check by yourself if the files were removed, by searching for atksgt.sys and lirsgt.sys files in the WINDOWS\system32\drivers or WINDOWS\sysWOW64\drivers folder, and you could even do a Windows reboot just to be sure the devices are unloaded from the running system.
If you have other games that use TAGES, and you did remove TAGES from your system, when those games are run you might get:
The game asking for the DVD or just stop/crash, meaning that you need TAGES in your system, so you need to run the TAGES installer (again) and install TAGES in your system.
The game automatically installs the needed TAGES files without user input so you end up with a working game, but with TAGES in your system too.
Remember that patch 1.5 removes the need for the TAGES code/files only from The Witcher.
Windows 7
When installing The Witcher on Windows 7 it will tell you that the TAGES client is incompatible with your operating system. This is fine as long as you upgrade the game to the latest patch.
If you run the game normally through the launcher, the game may crash after 20 minutes or a few hours of gameplay, most notably when entering/exciting areas or battles with many opponents. See next note to prevent this.
You will need to run the game by right-clicking the witcher.exe file in the witcher\system folder and select Run as Administrator.
Editing the permissions for witcher.exe permanently and then running the game normally through the launcher does not seem to work.
Saved games can be found in c:\Users\[[username]]\Documents\The Witcher\saves.