[Mobhack]

Presumably this did not happen before, and now without any changes to the game by you, it is?.

My suspicion then would fall on something like the installation of a new video driver or you have installed a new monitor that does not like 800x600 mode (in full screen or windowed).

The other error vector might be a video card that is in the process of going tits-up for some reason, e.g. memory problems.

Try 'desktop' mode and see if that works?.

Andy

NB - Some monitors these days will baulk at 640x480 full screen mode, but 800x600 should be OK.