Windows 98 restarting

If you done a clean format, that would eliminate the OS. There could be a possibility that it is a program you installed.
Whenever on those rare occastions I format, once I reload the OS, I install one program at a time, test it out, give it a goodrun through, and then go on and install the next. This helps me resolve issues I may face.
Another possibilty is it could be the battery. How old is the machine?

If the problem does not occur in similar software circumstances (lots of programs open, using a particular feature, etc) then the first places I would look are for heat related problems and a memory problem. I would open your case, remove, clean andreseat your RAM. If you don't have any method of heat monitoring, try putting the system in a cooler location, leaving the case open, adding a fan, etc to see if this affects the issue. Also check the voltage on your CPU if you can. Many of the Athlon system's I've seen autodetected at too high a voltage and were running too hot. Check your motherboard manual and CPU manufacturer for specifics. Close *isn't* good enough here. Make sure it's set exactly right. I'd also make sure your power is not related to the problem. Check the system power supply. Plug into another outlet and use a surge supressor (or better yet a UPS).

If this doesn't help, then you'll need to give me some more details: Hardware, software running (if any - does it reboot even when idle? Or only when idle?). Watch it carefully when it crashes and report every detail you can.

Good luck!