No fear, paleothol. What you need is a handy Linux bootdisk with a handy binary on it that will do the hard work for you. The one I'm about to point you to will enumerate the user accounts and let you modify their passwords. What I'd recomend that you do is set a new password for the Administrator account, login as such and add your user account to the Administrator group, and then log in as you. Make sure that you log in and out again as yourself so that your dad doesn't boot up to find the Administrator account as last logon!
Keep in mind, this assumes that he is using an account with Admin privs, but that isn't Administrator. If you change HIS password, he's obviously gonna know about it.
Anyway, this tool actually works with SYSKEY encryption now - pretty handy. Select the "reset NT Password" from the list of bootdisks. Enjoy!