Is the builtin Administrator account still available?
To access in XP Pro, press Ctrl+Alt+Del twice at the welcome screen. Use
Administrator as user name. Fill in password if one was assigned.
Otherwise, leave password blank.
XP Home, boot to Safe Mode. Username/password portion is the same as above.
If the builtin account is inaccessible as well, a repair install will
restore the default accounts and security settings:
How to Perform an In-Place Upgrade (Reinstallation) of Windows XP http://support.microsoft.com/default.aspx?scid=kb;EN-US;315341