How to learn an admin password

Alright, my parents are away on a 2 week vacation trip and I need to get into the administrator account so that I can do some updates for Java and such. The issue is my dad passworded the admin account. I know how to change the password, but he would kill me and I know how to do the F8 thing, but he has passworded the other one. I know that you guys often say well call the maker of the computer. I can't cause my dad rigged up this machine from old parts. So if I could get some help on this it would be great, because for whatever reason without the Java update my computer can't run applet windows. >.<

I am sorry for the long post but this is legit and I would like some help.

Yours truly Brianna