Computer Freeze when running video files

im getting rely bored with this problem now :S
i have had problems for months when running videos on youtube, or any other flash videos but only when i run them in full screen. even flash games on facebook wouldent run in full screen. my computer would freeze up and not even ctrl+alt+delete would work. i managed to fix this however. by disabling hardware acceleration. and thought all was fine,

but now i got the same problem again, i used a screen recorder to record and when i tried to play the movie when i was done, screen freezes again and i have to restart the computer.

so something must still be wrong?
any idea what it can be?

i have:

Win7 64bit

Motherboard: Asus M3A32-MVP Deluxe
Processor: AMD Phenom 9850 Black Edition
GFX Card: 2x HIS Radeon 4850 (in crossfire)
Memory: Corsair Dominator TWIN2X PC8500 4GB DDR2 (x2)
Skjerm: BenQ 22", CTX 19"