Power supply problem or hard drive problem???

How can I tell the difference between a power supply problem and a hard
drive problem? Sometimes the computer freezes up and it has to be
rebooted. Sometimes when the computer is started, it freezes when it's
booted up. After trying to boot it up for 4 or 5 times, it freezes
every time. Then if the computer is unplugged, left alone for an hour,
it will boot up and work ok for 4 or 5 hours. I think the power supply
is 300W. It's an old computer. It's a Micron. It's a Pentium 3, 733
MHz, 768 RAM, 2 hard drives - one 30G, other 20G. There's a CDrom, zip
drive, floppy drive. I'm running Win 98 SE.

When it has problems booting up, it seems like it lacks the energy to
start. I'm guessing it's a power supply problem.