posted earlier for very similar problem: as found here, near the bottom of the page:
"about the administrative accounts on Windows XP Pro and Home and all service packs. There is a hidden administrator account that has master privleges to all. to access this account: 1. Be at the welcome screen, 2. press CTRL+ALT+DELETE twice, 3. type administrator in the username box, and leave the password blank, it should log you on, if it doesn't and says the account is disabled, log back on to your account and follow these steps:
a. start>run and type CMD
b. type net user administrator active:yes
(this turns on the account, even though it is on, it is not visible in the user account control panel, so to set your password will be a little different. follow steps aa - bb to set the password, otherwise you may now follow steps 1-3 above.
aa. start>run and type cmd
bb. type net user administrator password here
(be sure the password contains no spaces- this will have set the password, you may now follow steps 1-3 above, except: type your password in the password box)
Patrick D., Chairman/Technology Director
Atomic Programming Incorporated
chairman email: firstname.lastname@example.org
Support staff: email@example.com
company email: firstname.lastname@example.org or email@example.com
Providing Technical Support, our own software
and games, other companies software and games, hardware, office tools.
HAVE A GREAT DAY!!!"
If it works, it might be good manners to email him a thanks.