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 pulled the CMOS battery. Shorted the pins. Waited 30 minutes. Popped the battery back in. Moved the jumper to normal and booted the computer. Same problem. I flashed the BIOS to the latest version. I was hoping it did not turn off during the flash. The flash was successful. Reset all the values in the BIOS. Rebooted the computer. Same problem. The hard drive is SATA. The DVD is IDE. I can't put them on the same cable. I will try a new cable for both devices. I downloaded the 'Ultimate Boot CD' and booted it. I ran several of the CPU checks. No errors. Frustrating! Is there a free bootable tool that can check the board?

 

I booted the computer last night to Safe Mode. It ran all night and I was able to access it this morning. Am I fighting a hardware or software problem?

 

I replaced the motherboard, CPU, memory and rebuilt the box from scratch. My resolution was a bad motherboard. I changed everything else.

 

Glad to hear all is well. I just read each of the above posts and after you said you tried a new power supply, my guess was the motherboard.....
:-D