Help, HL2

[fstime]

I just bought a 6800, I uninstalled all my drivers used driver cleaner, installed the card, then installed the latest drivers. Now when I try to play CS:source or hl2 my screen gives me an error then turns off and then I press CRT, ALT, DEL and turn it back on, end the hl2 proccess and im back to my desktop. Im assuming that hl2 is trying to play at a res my monitor cant support( I only have a 15 inch) is there anyway to force it into a res?