Graphic Problem

I got these problems recently and its driving me nuts!
1. every time i play a game (S.T.A.L.K.E.R for example), the characters have wierd streched lines
coming out of them, and it makes the game totally unplayable.
2. in addition to those lines, im seeing colored squars on the characters, sometimes i even get them 
on some parts of the game menus and on S.T.A.L.K.E.Rer's loading screen.

btw, I installed the latest GPU and motherboard drivers and it didnt helpd even a bit.

Specs:
Microsoft Windows XP Professional
Version 5.1.2600 Service Pack 2 Build 2600
Memory: 2,048.00 MB Crucial
GPU: NVIDIA GeForce 7900 GT/GTO (I HAVE THE LATEST DRIVERS: 93.71)
CPU: AMD Athlon 64 X2 dual 2.02 GHz 3800+

here are a couple of pictures showing what i discribed above:

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