This may work:
Log off until you see the screen with username icons. Then hit alt + ctrl + del (like you would at work) a couple of time until you get the username / password logon screen (like you would at work). Log on as the administrator with a blank password (unless you have set an administrator's password then use that). There is an account on Windows XP called the "administrator" which is usually hidden since you usually setup a new account with administrator's privaledges when you first start windows. Good luck !