restart not working

I just upgraded to XP by wiping my hard drive and doing a
new install. I have an older machine, a 900 Mhz Athlon
Thunderbird w/ 512Mb. The system previously had Me on it
and had many problems, but none of them involved
restarting. Now, everytime I try to do a "restart", the
system hangs. It shuts down most of the way, to the point
that it seems like the fan is running but nothing else,
and just stays there until I power off. It also seems
like if I turn the power back on too quickly, it will
return to that same state, but if I wait several seconds
before powering back on, it boots up fine.

Any ideas?