freezing

hi, first tread so not sure if im in the right forum i have a compaq desktop pc it has a q8200 intel quad core processor 4 gig of ram and a nvidia 9600gso graphics card my problem is 2,3 minutes after starting up going on the internet or games the computer just freezes i have to shut it down then restart it and then its fine all day my question is why does it freeze once then its fine.thanks