Win XP Pro SP2 rebooting upon startup. HELP!!!!

[chssvl]

This morning I turned off my desktop without any problems. It had been working perfectly.

However, when I came back from work I turned it on and during the "windows xp" loading screen it rebooted itself. Tried again and same thing. Went to safe mode and the same thing happened.

So I took my CD and I repaired the installation (the repair that reinstalls windows). The installation went fine but upon first boot up it rebooted itself again and this time even before the win XP loading screen showed up.

What can I do??? Help!!!!

[Statica]

Can you boot to safe mode? Did you have an antivirus running in the background before? Or have you installed any software the last time you used your computer?

[chssvl]

No to all questions. I hadn't installed any software. I didn't have any anti-virus software running in the background although I did have Zonealarm.

Haven't even been able to start in safe mode. Currently reinstalling from scratch but I jst can't understand what happened

I left it on last night as I was authoring a DVD and downloading the latest 3d mark. This morning before going to work I checked my e mail, checked if the authring went fine and turned it off correctly. Then when I turned it on today it just rebooted.

I already removed every PCI card, checked every memory stick independently, re-flashed the motherboard's BIOS and still the same *sigh*

[chssvl]

Just finished re-installing from scartch and everything seems to be working fine now. (though I'll have to reinstall everything all over again ).

I just don't get it.