Since it's XP Pro you can press CTL+ALT+DEL twice while at the login screen and you will get the classic login box. Enter Administrator in the User name box but leave the password field blank since you didn't set one. That should log you in as Administrator where you will be able to make any changes. If you are the only user, so don't normally get the login screen, just choose Log Off from the Start button and it will take you there. Having said all this, if you are the only user then you would normally have administrator rights anyway. Go to User Accounts in Control Panel and users are listed along with the type of account under their name.
What Eric surgested will noly work in safe mode this account remains hidden otherwise
at imidate boot of pc hold F8 key and wait for a B/W doss screen tell the pc to boot in VGA OR DOSS mode and wait at your login you willc see and extra account called ADMINISTRATOR click it and you can logon as an administrator from there crate your own account with administratoion settings and restart your pc you may be asked on another B/W screen how to boot say "Boot windows Normaly!"