PC Turns self off, on its own accord.

[Windybey]

Just after i posted my last message.
The pc just turned to a black screen and rebooted.
It had been on for about 30 mins.
it seems when so many resources have been used, it just falls over and reboots itself.
Running Quake 3 will kill it in 30 seconds.
help someone please :~(

 

[littleprince]

didn't catch ur last msg....
more details about your comp pls!!!!!!!