it could be the power supply. was a it a generic PSU you got bundled with a cheap $50 case? give us the make/ model of it. you can find it on the side of the power suppy in text.
can you describe the symptoms more, is the computer restarting on you as soon as you start the game up or does it take a few minutes of playing before it restarts? is there any other situataions where the computer becomes unstable or restarts on you?
if it does it right when you start up the game that is probably due to a driver/ software issue. if it takes a while that could mean the video card or power supply is overheating/ overloaded.
as far as drivers go i don't know anything about which drivers/ updates you should have for that radeon card or the counterstrike game. but please list the driver version anyways
if someone is familair with this video card/ game we would appreciate any advice