This!9 times out of 10 when you have a problem after you suddenly shut off the computer it's because you caused the HD to have a bad write to a file. IE it was writing to a file and you shut it off right in the middle of it, causing a corrupt file. In this case, a registry file. All that needs to be done is run a checkdisk and it should fix the file however, you need an operating system disk to do that. Borrow one from a friend. doesn't matter what version as long as it's xp
boot to the disk
press r to go to recovery console
type chkdsk /r
profit.
chkdsk will fix it.