Certainly sounds most likely to be a power supply and/or CPU fan problem. Could possibly be a power management problem, but that seems unlikely to me. I have seen these sort of symptoms on a system where the CPU fan was not working correctly. It can also happen if the power supply is starting to fail, and can't support all the cards in the PC, so the advice from mooky about running a minimum configuration is a good idea.
By reseting the jumpers on your motherboard, you should have reset the BIOS back to its default settings, so that shouldn't be the problem. To make absolutely sure, there should be an option in the BIOS to "restore default settings", or something similar. This will ensure that e.g. the CPU is not being overclocked.
I note that you said that "I have looked in the bios at the temp of the CPU and it looks OK. I know that if it gets too hot, it can cause a system shutdown. "
One thing that could be significant here, is what happens if you leave it in the BIOS screen for more than 20 seconds or so (a few minutes would be a good idea)? Does the CPU temp change? If not that would seem to imply your power supply and CPU fan are probably OK.
Another thing to try is to set the BIOS to boot directly from the CD (if you have a Win98 CD), or from a floppy which has a standalone boot of Win98 or DOS. If this works then you probably have some type of virus on the HDD - if it still shuts down after 10 seconds or so, then it is definitely a HW problem.
