Ok
1. If you look through the BIOS settings (when it boots there should be a notice telling you which button to hit to enter setup, it's usually 'F5' or 'Del' or something like that) you'll find the option to turn that boot password off.
2. Re-registering the machine is more tricky and is going to involve messing around with the registry, something not for the faint hearted. If it were me I would completely wipe the drive (the fact that it's still 'registered' to someone else tells me that all their data will still be on there anyway, and you don't want that hanging around if you can help it) and reinstall from scratch. Unless you've got the discs that came with it that could be a bit of a pain though
