My computer keeps restarting at a specific point..

I have a homemade system with an AMD 2500+, 768 DDR RAM, ATI 9800 Pro, and a A7N8X-Deluxe board. My problem is that my PC restarts right before the welcome screen.

I think it is the power supply. The rails are running very oddly. At the BIOS it shows that they are at:

12v - 12.86v
5v - 4.85v
3v- 3.13v

And my CPU temp is like 59C-62C and it used to originally run at 42C-45C on high load.

I dont know what to do. When I try re-installing Windows XP, right after it loads all the files I see the whole "press enter to continue" screen and then a second or two later, it restarts. It's weird. I have some random generic 550 watt PSU. I was thinking of getting a new one but I want to figure out the problem first. Thankx to anyone who helps. I really appreciate it.