Need help w/ Random Colors on Screen in Games?

I've got a an AMD Athlon 2800+ (2.8Ghz), 1 gig RAM, & a Geforce 6600 GT vid. card.

When I play different games (DoW, C&C3 demo...) I keep getting shades of different colors in the background that flash all over the place... mostly green for some reason.  The game usually starts off fine, but the more I play, the worse it gets.

Is this a video driver problem?  Anyone have a good link to download the updated drivers if it is.  Any other issues my computer may be having?