Ive seen this about 3 times in the last couple weeks.
Reading What everyone has said, seem this case is more difficult.
my thought is booting to safe mode, Use "administrator" instead of admin, i couldnt tell you off hand what is available at safemode log in. Anyway ive learned(xp pro)that Removing the password Through Safe mode (f8) is possable. In order to do so effectivly you need to only REMOVE the unknown password. Then set it with the actual account of the unknown password.
It works, i hope this is of some help.
