Whats wrong with my computer?

[Barry_G]

I bought my mom and dad a computer. Cheap one $800 with a $400 rebate. 2.6 ghz, 40gb hard drive, 256MB ram, cd burner/dvd-rom. DSL connection to the internet.

They use it for e-mail and playing games on MSN.com It has norton firewall, up to date virus definitions etc..

After 10-15 minutes of use it will randomly turn off and then restart.

I've ran ad-aware and spybot and it finds no problems. I've reformatted the hard drive and installed windows xp pro.

I bought a ups today and that was no help either. The computer is about 18 months old and the warranty is expired.

Any idea about what could cause this?