Random black screen

Hi everyone

i sometimes experience a random crash once i login to crossfire. i can get past login screen but at anytime from the login screen onwards the monitor may just go black and i cant use ctrl alt del or alt tab or anything. the only way to fix it is to turn the computer off by holding down the power button. ive got crossfire and xtrap and that exlcuded by avg free. its annoying because it hardly ever lets me play so i rarely play anymore just because of the chance that its going to black out.

does anyone else have this problem? and is there a known solution?

PLEASE HELP ME!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!