if its xp I believe F8 on start up and reset/delete admin password.
oooooooor, the more logical solution, just download the drivers for the printer from the manufactures website, and later ask your brother to add the printer to the family network. no cd is needed, then either way.
edit - or if you don't like ur brother, pull his motherboard battery and replace it about a minute later this will solve all future issues with this computer.
you should be able to add guest login to the computer as well, may be safer than reseting admin, as there could be a default password to crack (don't recall this ever happening to me though) but guest doens't start with a pass and should be much simpler.