You didn't tell which XP but the spare admin account when we boot safe mode let's us create a new account and more.
Let's read this article for more details -> http://support.microsoft.com/kb/811151
Tip: Consider your machine a "workgroup" computer.
I have a Toshiba laptop running with Windows XP. When I click to log on, the desktop comes up, but it immediately logs me back off. It doesn't give time enough for any of my icons or the start button to appear so as I can click on anything. Hope someone can help!
Thanks in advance!