Crossfire problem

well, i have a problem guys,
when i turn crossfire on, everythings is ok,but when i go in game the screen goes black and computer turns off..

dual core 2.80ghz
2.5gb ram
windows 7 32bit

I have formated my pc, i have reinstalled it i have done everything, but nothing seems to work, thanks
and please help

i have never seen a problem like that before but hope you get it fixed