Black screen in several games.

I'm suddenly having problems with several games, they launch, but I get a black screen .
The following problematic games are: Crysis, Crysis Wars, and S.T.A.L.K.E.R Shadow of Chernobyl; I can override the black screen on Crysis and Cr.W by ALT-TAB, they seem to start in windowed mode, then ALT-TAB again to fullscreen to continue playing the game normally in full screen. S.T.A.L.K.E.R ShOC completely freezes, I have to end it manually thru task manager. I simply don't understand what could cause it; These problems started yesterday night, without any reason, I wasn't fiddling with anything, I decided to play Crysis some more and the 3rd time the game wouldn't start, I checked all the games; Far Cry 2, Crysis Warhead, S.T.A.L.K.E.R Clear Sky and CoD 4 start up just fine; I've done a system restore but nothing. Before I take any steps further (like reinstalling the games or reformatting) I'd like to know what are the possible causes for this.
Thanks in Advance