Some things to try:
Leave it unplugged without the battery for a few hours. then try plugging it in and booting.
See if you can get into the Bios. Try saving and exiting followed by returning to defaults. (if that doesn't work)
Boot from a floppy or Bootable CD. (also might remove HD and try this)
If you can get into safe mode you can try a defrag, cleanup, virus scan(possibly) and you have access to the recovery console through the XP disk.
Good luck.