Flickering Display

Whenever I start some games, typically FPS (and I guess running w/ openGL), the display is all messed up and flickering, with colored polygons and lines all over. I guess I must have done something wrong, since some of these very same games used to work fine before (so the game itself is fine)… For ex, Aliens vs Predator 2.
I tried everything, and updated ALL the drivers. I also checked all the display properties.
It isn’t another electrical device’s interaction, neither a refresh rate pb!!!
Please, someone care to help?

Windows 2000 Pro
Athlon 2000 xp
GeForce 3Ti 200 128 Mb
DirectX 9