This is a known bug with a fix. It's been a long time since I played the game and visited the forum so I don't exactly remember the details. But I run Windows 7 Ultimate 64 and the game runs fine. I also can confirm that it runs fine on Vista Ultimate 64. Go to the official The Witchers forum to check for a solution.
http://tw1.thewitcher.com/forum/index.php?board=14.0
Here's the question I asked regarding the proper order of installation to play the game.
http://tw1.thewitcher.com/forum/index.php?topic=30402.msg5444714#msg5444714
As you can see, you are supposed to remove Tages.
See here for more info.
http://witcher.wikia.com/wiki/1.5\_patch#Download\_links\_.28Patch\_1.4\_.E2.86.92\_Patch\_1.5.29
Alternately, you can go here to remove Tages.
http://www.tagesprotection.com/main.htm?page=minimum.htm
Does the system error bar you from operating the system properly?
I hope this helped. :)