Personally this is what i would do.
Unplug from wall
Open Case
Take out cmos battery and disconnect all case plugs from the motherboard (like reset, HD, HD light etc...)
Plug all those cables back
Restart PC without the cmos battery in
then shut the machine down once the windows boots up
Put cmos battery back in again.
(I would also try this, once you've followed the above steps, dont plug in the reset plug, and try booting and restarting.)
Boot up machine and restart, if that does not work, i would say you have a fried circuit somewhere, i doubt this has anything to do with XP.
I would also check to make sure the PSU is on 115v and not 240v, although i'm sure you would have noticed by now.
The other thing you can do is take out the memory and alternate between the different sticks being in the mobo one at a time, just to check them out, test by booting up and restarting.
Alternatively, you could install ubuntu on a small parition and restart that, if that does restart fine, then its clearly and xp isssue, if not then its hardware based.