Do you know anyone who might have had physical access to your computer? (little kids, a neighbor, anyone really)
My first bet/guess is that one of them signed on to one of your accounts and passworded it. They either did it accidentally or thought it was "cute" or whatever the heck else people are thinking these days.
If you can still get in admin account, just do what those other people said, but, if not, I'd suggest finding out who did it and getting the password out of them.