[HELLBEAST4]

Is there a cvar for fullscreen?

like seta fullscreen "1"

beyond checking that out I would try to autoconfigure and try again.
if not then i would go into the main menu i would switch resolutions in the hope that switching would revert it back to fullscreen mode.

beyond that, i would reinstall my video drivers and stuff(i've actually had this problem before, but it's always powerstrip related for me).

you could also tone down hardware acceleration on your video card, but reinstalling the drivers should clear up any problems with them.