The Witcher runs fine on Windows 7, after you have done these two things:
1. You need to download and install the 1.5 patch for the game.
2. You have to download and install the TAGES (copy protection) update. The TAGES drivers can be found here:
TAGES drivers Click on the Drivers tab at the top of the page and then either download and run the Windows 7 64-bit drivers or the Windows 7 32-bit drivers, depending on which Windows 7 you have.
The 1.5 patch also allows you to play the game without the DVD.
After doing these two things, the game should run just fine.