Just to be sure. The original administrators account? Once you add an account it will likely become a hidden account. To unhide it read this http://www.pctimelimit.com/how_to_enable_administrator_account_windows_xp.html
If that doesn't work then you probably do need a reinstall or to erase all passwords but forum policy is that we are not to help circumvent security so I can't help you. Any good techie could help you with that should you choose that route. Always remember for security, passwords are like locks, they keep honest people honest but the locksmith can bail you out just like the thief will scoff at a lock.