random shut down when booting

So when I was at work the other day my roomate heard my computer making funny noises so he turned it off. When I got home I had a look at it and determined that the power supply was going bad. It was making a strange grinding noise and was generating a lot of heat. I replaced the power supply.

Now here is my problem. When I go to boot the system it randomly turns off during the boot process. It never gets to Windows, but usually gets through the initial RAM test and detecting the IDE drives. I figured something got too much heat when the power supply went so I've tried booting it with many different configurations. I've tried with one of either stick of ram I have. I've tried with either of the two hard drives disconnected. I tried with the video card removed. I've visually inspected everything and I don't see any visible problems such as burnt circuits. It's not a lack of power the new power supply is a 550 and the old one was a 500.

I think it is probably an overheating issue, but I'm not sure what would be causing it. If i try to power the system on again right after it stops it cuts out almost immediately.

My specs are as follows
3.0GHZ P4
Windows XP SP2
2GB ram
2 250gb hdd
ATI Radeon x1600 video card
Sound Blaster Live! sound card