How did you make the start bar look like OS X is what I meant.
Did you enable fire sharing and discovery? Are you able to view files on your brothers computer and is he able to do the same?? Vista should pick up your brothers computer if he is sharing some sort of files. Your brother may need to enter a user name and password to gain access to your computer. There is a remember password option so that will not have to be done all the time as well. What is the workgroup his computer is on and yours? XP home defaults to MS HOME and XP PRO (and Vista) defaults to WORKGROUP.
You should really be able to connect to each others computer with the game just as you did before.