Computer Spontaneously Reboots Itself

I have an older computer running Windows 98 that spontaneously reboots itself. I just recently fdisked the hddrive and reinstalled Windows 98 and other programs; also, did a complete virus scan of the hddrive. I was thinking it could possibly be mypower supply but I don’t know how to test that…

…OR if there is any other ideas of what could be causing this to happen and ways to fix it, I’d really appreciate it!!!

Thanks!