Winodws reboots after login screen

[masterbs1]

I just used my wife's computer yesterday and it ran fine. Now when booting into windows it gets past the windows logo screen and then reboots. I can't even get in during safe mode.
I think I still have Service Pack 1 on it. Any ideas on what might be causing this?

Xp2200
Soyo KT400 Dragon
512 PC2700
FX5200
350 PS