Computer not booting!

Ok so I turned on my PC today and it gets past the BIOS screen and stuff fine.

The Windows XP boot screen comes up and starts loading, but after a few seconds the screen goes black and the computer reboots…wtf!!

Just before the Windows boot screen comes up theres a thing that says something like ‘Your computer did not shut down properly’ or something and then has a few selections like safe mode and start computer from last know config that worked.

It won’t even boot in safe mode, does the same thing for all the selections.

Whats wrong with it??