Computer shuts off during boot

I built this computer for my wife back in July 2007.
Specs: WinXP SP2, Abit NF-M2S, AMD Athlon 64 X2 3800+ 2.0GHz, Wintec 1GB RAM, WD 80GB HDD, onboard video and LAN, floppy and DVD drive. The power supply is a CoolMax CM-300.

The computer started to crash randomly about a month ago and has gotten worse. Now it dies during boot up. I ran MemTest and the RAM was failing. I bought new Crucial RAM - 2GB. I ran MemTest for 2 hours about 3 passes and no errors. The problem still exists. I can get to Safe Mode command prompt. I ran msconfig and selected the diagnostic boot up. No drivers or anything is loaded. Normal boot up it fails. On the screen I get the XP splash and a few runs of dots then click. It's off. I took out the hard drive and put it in my other machine. I ran scan disk and found no errors. I can read all the data with no issues. I bought a new hard drive and started Setup. It runs about half way and then click. It's off. I took a power supply out of another working PC and the problem still exists. I shorted the BIOS pins and reset the BIOS. I booted and loaded defaults. Problem still exists. Boots up half way and then click. It's off.

Is my problem the motherboard or CPU or both? or???

I changed the motherboard and rebuilt it to fix the issue.