Computer randomly shuts down

Hi everyone,

I'm having a bit of trouble with my computer which seems to randomly power down. It all started last night when I left the room to grab dinner. Upon my return, I found that my computer had mysteriously shut down. I turned it on, got to Windows, and it powered down without warning. I switched it on again, and this time it powered down even before Windows had loaded.

I assumed that the problem was due to my PSU, so I went out to get a new one this morning, higher wattage and all. Plugged it in, turned the computer on, and got a good 15 mins out of it before it shut down once again.

After doing a bit of research on the web, I thought that the problem might be due to the CPU overheating, so I switched my computer back on going into BIOS to monitor the temperature. I watched it rise steadily for about 20 minutes till around 105 degrees C before it shut down. I thought I had found the source of my problem. I switched back to my old PSU since I discovered that it was unrelated to the problem and proceeded to clean all the dust out of the computer's case. Started the PC again, monitered the temperature. It slowly rose and stayed steady at around 52 degrees C this time but shut down after about 5 mins or so. So it isn't a temperature problem.

I'm sorta lost now and don't know what to do. I can't even get in to run a couple diagnostic programs. Any pointers? Any help or advice would be greatly appreciated. I have a P4, 2 GHz with 512 MB of RAM (I think) and Win XP installed. I have two HDDs, a CD-RW and DVD-RW. Thanks in advance!

Glandith