Toshiba Laptop Restarts when idle

I have a Toshiba Satellite Laptop,P4 3.6 GHz, 1GB RAM, Win XP Pro SP2.
The computer is fine if it is used. When left idle for more than 10 min it restarts by itself.
Here is what I’ve done so far:
When left idle in Safe Mode it doesn’t restart.
I cleaned it from dust, all fans are spinning, doesn’t look like it is getting really hot, and as I mentioned it will restart only when idle. It will also restart if left at the user login screen. Anti-Virus is up to date, and I’m running full scan in safe mode right now, but I doubt it will turn anything.
The only new driver I have installed was a printer driver, which I reinstalled (from HP site) with the problem still there. Any ideas?