IIRC there is a Microsoft work around for this. (I know because it happened to me). Basically, it's easy if you have a guest account. I can't remember the exact details but in the guest account you can get to the control panel and then change the password on the protected account.
It took a bit of searching on google and Microsoft, but it took about 30 minutes to fix.
Try here if it's a Vista OS
Administrator Account - Vista Forums