do you see an error message at some point?
describe in which part specifically your computer restart?
what were you doing last before the computer started acting up on you?
it may be a driver problem:
have you tried to restart in safe mode?.. after the post sequence press f8 and start the computer in safemode. there are other options you may want to try from the advance startup menu that comes up when pressing f8. if you are able to start the computer in safemode your problem most likely is with a driver.
did you install new hardware?
try uninstalling the new hardware and restart the computer to see if that solves the problem
you may have a problem with some configuration files or booting files...
try to restart your computer with a windows 98 boot disk. first download the disk from http://www.bootdisk.com/
and make a floppy disk as directed. then get into the cmos/bios of your computer and make sure your first booting device is your floppy drive, then put your newly made win 98 boot disk in the floppy drive and restart your computer. at the prompt type c: then enter then type win