Either set two user accounts in windows one for you and one for him and they will be kept totally separate, or setup 2 playlists in itunes and only sync the your own playlist to your own ipod.
Yes, though if you use the two user accounts method you do have to be a little careful not to end up with two copies of all your music on the computer, so you need to make sure both users itunes is looking in the same place for the music.