sorry so late in responding, but you are in ok shape, as long as your friend didn't put a master password on the pc.
You need to get your OS install disc, and put it in the drive. Boot to the cd by holding C while starting the computer. It will come up with a menu, looking like it's going to install the OS.
Look at the top, and there's a "utilities" menu. Click on that and click on something like "reset password." It'll reset the admin password with whatever you type, just as long as someone hasn't set a master password.