If it was a stolen PC, it would be more efficient to just wipe it clean and install a copy of XP over the top.
Working in a PC Clinic there are numerous occasions where we have to do work on a customers PC but can't because they haven't left their passwords. XP Home is not a secure OS like Windows 2000 / XP Pro.
Just load into safe mode, and on the welcome screen is the Administrator account. Use that to remove the users password. Note that it will also wipe out any saved passwords for webpages etc.. that they may have on the system (security precaution!).
And for anyone who thinks I just told him how to gain access... all he had to do was boot into safe mode as admin anyway, and he would have complete access to the system. It's just like pressing Cancel on the 95/98/ME login screen.
Oh, and if the Admin password has a password (if its an OEM it won't), then you're stuffed :p