You should still have one administrator, but it may be hidden so there a a couple of solutions depending on your XP version. Once you logon using that administrator you can elevate your user status back to administrator.
If it is XP Home there is a default administrator account that only shows up in Safe Mode, so boot up in Safe Mode and use the default administrator account to open users and elevate yourself back to an administrator.
If XP Pro try hitting CTRL-ALT-DEL twice at the Welcome screen to see if the default administrator account shows up. You can also change a registry key to make it show up if that does not work. Just let me know and I can go through that.