My computer is about 7 years old. Its has an Asus A7N266-VM motherboard and an AMD Athlon XP 2400+ CPU. My problem is that my computer suddenly and randomly switches off on its own. There is no blue screen, no error messages, the pc just goes off as though somebody pulled the plug. The motherboard power indicator LED remains ON. It might happen anytime from within 5-10 mins of starting up, to maybe an hour or two later. Sometimes it works perfectly fine and never turns off. And once it shuts itself off, i can't power it back on. If i press the power button to start the computer, it turns off again in about 2 seconds before even the POST can start. If i pull the plug and try again later or shake up the chassis a but and retry, it might boot up again and run for some random period of time before the same story repeats.
Another (maybe) related problem which i have been facing for a longer time is that my computer cannot be shutdown. After windows(xp) shows "Windows is shutting down..", the system powers off and promptly restarts. So all I can do is pull the plug at the right instant!