"Grate effect"

[Giger]

Hi, when i play the game, there's this weird glitch with the glowstick, as everything appears with black squares (image in attachment) and i can't figure out the cause:

i'm using windows XP professional, a 2.8Ghz processor, 756MB ram, and a Geforce FX 5200. I have all the latest drivers and the 1.0.2 patch for Penumbra. This glitch seems to be most apparent with the glowstick, although i can observe it with the flashlight too. I started playing the game with the default graphics options, then i saw this and put everything low, and then later, everything high (you never know it could have worked)

If you can help me i would be very grateful