If you have the XP cd:
http://pubs.logicalexpressions.com/pub0009/LPM Article.asp?ID=305
(don't worry - it won't be a fresh install)
If you don't have the cd:
Windows XP Home does not let you log in with the Administrator account normally. To access it, you must restart the computer in safe mode, a special 'minimal' mode used to correct errors caused by third-party software, among other things.
To enter safe mode: Restart your system. Immediately after the POST screen, press F8 a few times to bring up the XP advanced options menu. Select the 'start Windows XP in safe mode' option.
Once safe mode has loaded, login with the username 'administrator.' The password is blank by default. You can now change user passwords by going to 'start\control panel\user accounts.' Reboot normally when you are done.