That generally means that an important system file has become corrupted for some
reason and the standard fix is to run a "repair" installation from the Windows XP
installation CD, but that will be no good if the disk itself is dying. Does this
happen when you try to logon, or can you get some sort of sense out of the machine?
Will it boot into Safe Mode? If so, there may be one or two things to try first to
see if it can be recovered. Cheers. Spexx.