Sounds like the registry is corrupted. According to the Microsoft article, "How to recover from a corrupted registry that prevents Windows from starting" --
http://support.microsoft.com/kb/307545/-- a common cause is powering down (or a power failure) while the system is writing to the registry, which leaves it in an "undefined state". Sounds like what happened here. If you have a Windows install disk there is a procedure, outlined in the article I linked to above, which you can try. However:
* It is not for the faint-hearted. Registry repairs are serious stuff, and the system could get even more trashed. Read the article thoroughly first and note the remarks about NTFS and FAT32 at the bottom of the page. If you do not consider yourself ready to do this, there is no shame in calling a repair service!
* It is not for OEM Windows installations, that is, one created by the supplier of the PC, (Hewlett-Packard, Dell, Compaq etc)
* Or you could consider reinstalling Windows if you can tolerate possibly losing stored data.