hidden computer administrator

i have been given a laptop, running windows xp home, with sp 3,
there is one user, fred, limited account ,
i have tried to set up a new account , as an administrator , but not allowed,

have discovered there are 2 admin accounts already, computer admin, and 2nd admin user ,they are hidden accounts
how can i remove them all, would like to format drive and re install, so i have a fresh laptop and use win xp pro

but cant even do that, says dont have permission

any ideas would be helpful

thanks
robert