Graphics Glitch

[924]

Hello. I registered in this forum to see if I could solve this 'little' problem that's making my game unplayable... I reinstalled Penumbra after finishing it a couple months ago so that I could play it again from the beginning, yet this time around the textures (or is it the shadows?) have a major glitch, a black rectangle keeps popping up and flashing in the upper half of the screen... Here are some screens of the problem:

http://i59.photobucket.com/albums/g283/k...-55-82.png
http://i59.photobucket.com/albums/g283/k...-19-54.png

I am running the game in a Pentium IV 3.0 with 1gb RAM and a GeForce 7600GS
The game worked fine before and the only difference from the last time I installed it is the OS, before I had Windows XP Service Pack 1 and now I upgraded it to Service Pack 2... Do you think it has something to do with it? I've installed all the patches for the game, I tried turning everything off and setting the graphics to a minimum, I tried reinstalling the game, yet everything to no avail. What can be wrong?

Help is appreciated, thanks in advance.