Hey Seph,
I dunno if this even comes close to your problem over there (yours seems to be far worst - sorry
)... But did you take notice if before the system would shut down on it's own, if you received a BSOD? This had happened to me when I went to reinstall XP on my parent's computer... I dunno why this would have happened, but after moving the RAM around, things seemed to work again...
If the system keeps cutting off on you when trying to log in - I don't think it would be a PSU issue... Did you try to, maybe, disable all the onboard stuff you are currently using? You should pull everything and start with just your CPU, vid card, RAM, and HD - and see if you can get in.
Can you try to get into safe mode to view your event logs?
Hope something I mentioned helps you out a little...
Good luck,
-SlickVic78