It depends on what password you mean.
The easiest way around this is to ask your brother for the password.
It is possible to reset most software passwords but I have no intention of posting on an open forum how to and I hope that no one else will either as it is a recipe for even more stolen computers.
If your brother can't help (and I do not see why he can't) take the PC with proof of ownership and ID to a local computer shop and they should be able to help you.
V, January 2009