Power supplies

Old power cord could be culprit. Try a new one and see what happens.

Connectors get loose after many ins and outs and cause intermittent power glitches.

Make sure that all internal connections are properly seated.

If this is a socket A (AMD Athlon/Duron) system, enter BIOS setup, and manually set the CPU speed to the proper settings.

If the system is more than 4 years old, check / change the CMOS backup battery.

Chas

Ive learned to live with that same problem for a long time now. Ever since I first started using computers I noticed that the problem can occur seemingly for no reason at all. Im sure that the problem lies in the powerbox itself but as it can be very easily fixed I have not done much reasearch into the subject. If you were to cause a short in the systems powerbox like I have done by mistake on several accounts, you will find that you will most likely have to switch the plug before being able to get it to run again. Ever since I have upgraded my powerbox to one of better quality with a larger power cap I have not noticed that problem nearly as much or at all for that matter. I could be wrong but as I said I have not done much research into it, its just my opinion from personal experience.
Hope this helps

make sure the wattage of the PSU is high enough 2 cope with the system...

AMD's tend 2 take more power (cause there crapper) and it all depends on things like how powerful the graphics card is, how many fans are in it, how many PCI devices are in, how many drives etc...