Computer turns off, reboots randomly..

[Alia Vetinari]

Around three months ago, a friend (since I was unable to at that moment) bought some parts to build me a computer. It has an AMD Athlon XP 3200 processor, Asus A7N8X-e motherboard, 512 MB of RAM, 160 GB Seagate HD, an ATI Radeon 9200 256 MB graphics card, DVD burner, and then simply a 56 fax-modem, plus the ISDN modem I use to connect to the net.
At first it worked more or less fine, expect that I still had to use Windows 98 SE for a while and it didn't seem to like the HD or the RAM much ( too much RAM for Win98? ). Finally I manage to get a Win2K copy, and at first it worked fine.. then suddenly it started rebooting randomly, after several hours of use sometimes, other times just after around ten-fifteen minutes, and it also gave me random error messages (Win2k blue screens). I checked some of those error messages and they do in fact seem random. The reboots were more normal than the blue screens, actually.
I thought it might have been the RAM, so I opened the case and changed it to a different slot. For around one week, during which I played the World of Warcraft beta, it worked perfectly fine. Not reboots or anything, and I used it a lot. Then, a day after it ended, and I returned to using the computer in a normal way, it started turning off. The monitor would turn off, and so would the computer (the fans stop spinning, it doesn't read the HD, and it's completely silent, so I guess it does turn off), except that the 'Power' led stays on, and I have to press the button to turn off the computer for around 4 seconds so the led turns off as well, then press the button again to boot the PC.
I have no idea (neither does my friend) about what it could be. All the parts are still under guarantee so I could replace them. I already tried using a friend's RAM and graphic card, and it keeps doing the same.. so I'm guessing either the power supply is dying, or the motherboard is defective? I can't seem to find anyone with a similar problem
Thanks.

 

[Lorand]

Could be overheating too... Have you checked the temperatures?
I think the RAM is ok, since you already tried to put different memory modules in the system. But defective motherboard could also cause memory errors. To check if the mobo is causing the problem run memtest.

 

[Alia Vetinari]

Could be overheating too... Have you checked the temperatures?
I think the RAM is ok, since you already tried to put different memory modules in the system. But defective motherboard could also cause memory errors. To check if the mobo is causing the problem run memtest.

Temperature seems to be fine. It's one of the first things I checked (forgot to mention it..damn).
I ran the memtest program, it ran fine, not giving any errors, until the 7th test. When the 6th test ended and number seven is going to start, it turns off, leaving the 'power' led on. I did it twice to make sure it was that.. and yes. I had enough time to see it say it was the 'random number' test (I think).
Another thing, apparently it'll let me do a defrag, but if I try to use the "Free space" thing Windows has when you right click on a HD, it'll turn off after a couple of minutes, without finishing the process.