bizzare startup/restart problem

I've built my new system and it's fairly stable, I'm very happy with it when it's running, but here's an odd problem I'm trying to find a solution for.
On startup, the computer boots normally, goes through the windows XP "welcome" splash screen, starts to load my user account and then *blink* it restarts... If I don't touch anything it will just continue in that restart loop just after starting to load the account. Here's the REALLY bizzare part, if I start throwing commands at the computer just as it starts to load my account (moving the mouse, opening the start menu, etc) IT STARTS NORMALLY and is perfectly stable!! 8O

Bizzare yesno?

Has anyone seen a snag like this before? Does anyone have any suggestions?

Thanks