How can I change or delete the "Authority" on windows Vista? I bought a used computer that is almost useless!?

I bought a great deal HP laptop used from some guy on Craigslist. When I got it home and tried to install my printer, I got an error message telling me that I did not have the authority to do that, and to contact my network administrator. Later on, I tried to install a game for one of my kids, and got the same message. The computer is almost worthless since I don't have "Authority" on this network, and since I bought the computer used, I have no idea what network is even involved! Someone out there in cyber-land knows how to just either override or delete the whole admin. settings so I can start fresh with this computer and have a workable machine that can play games, print things, and hook up to my other desktop via wireless router (I don't have the authority to do any of these things currently)...Please help! Thanks so much!