Windows Xp rebooting, but only once.

[Hodsocks]

I have a strange problem with a machine that reboots itself after a few minutes but once it has rebooted it runs Ok. The various error messages that pop up suggest a driver problem. one even identified a Realtek sound driver problem. I updated some drivers off the Microsoft update site and downloaded the latest driver from Realtek, which I know has worked well on other Win XP machines.
If the machine rebooted repeatedly I might look at hardware problems but as it only reboots once it has me puzzled. The only thing that has changed on the machine recently was that I reformatted the drive and reloaded all the software.
Any ideas anyone?

 

[smore9648]

What error messages are you getting?
What antivirus, firewall are you running?
What programs are you running?

 

[Hodsocks]

This problem occurs if just windows is running, if you boot up and leave it for a few minutes it will reboot. Any blue screen that appears refers to a driver being the most probable cause, as does any feedback from reporting the problem to microsoft.
Norton internet security is installed.