5850 Black screen when loading full screen any game

I hope someone here can help me because I'm at the end of my rope here!
I built my system myself a couple of months ago. It worked fine until about a fortnight ago when it started taking a long time to load full screen games. During this time I would see a black screen on both of my monitors.
It got worse and now I can't load ANY full screen game without crashing. I see a black screen and the PC is unresponsive even if left for a long time.
I've got the latest drivers and have even installed older drivers, but I get the same result.
I haven't got anything OC'd and the system is cool (typically not exceeding 35C CPU and 50C GPU)

My system:
AMD PHENOM II QUAD CORE 965 AM3
ASUS M4A79XTD EVO/USB3 MOBO
CAVIAR GREEN 1TB 64MB 7200 SAT
WESTERN DIGITAL 80GB DRIVE
WESTERN DIGITAL 250GB DRIVE
G.SKILL RIPJAW 4GB (2X2GB) DDR3
SAPPHIRE ATI RADEON 5850 1024MB
ZALMAN ZM600-ST HIGH-EFFICIENCY -- 600W PSU

This happens with any full screen game: Dead Space 2, Cities XL, Sims 3, and even older games such as Battlefield 2142.

Any help would be very much appreciated!