CP freezes when unattended for few minutes

Hello,
My computer freezes when walk away for a few minutes, or when unattended, even if is playing music.
It goes in like dorm status, dark screen and even I touch the key pad or move the mouse around it does not come back, I normally have to kill the power and restart the PC, but this is not normal because it does more and more often now.

I have an antivirus, and run periodic scans. It runs in Windows Vista.

Any tips on how to fix this problem at home?