Surely if it was your PC you should have made yourself the administrator, AND set a password that nobody knew.
If you were going to allow someone else to use it you should have set up a separate user account for them with limited user rights (easy to do with Windows XP). That way they could NOT have set themselves up as an administrator.
But late now I know.
It is possible to have more than one administraotr on Windows XP so are you sure no others IDs (if you have any) have administrator rights ?