check the Firewall settings to allow iTunes thru (Sys Prefs > Sharing )
although iTunes will prompt you to do it anyway if it's not, when you turn on sharing (iTunes > Prefs > Sharing).
They have to have iTunes running on their machines to see and play your shared library,
and they can't copy files thru iTunes, you have to go thru normal file sharing for that (again check the Sys Prefs > Sharing settings and turn on Windows sharing so they can see your mac under their windows network). They'll need you login and password to access your Music folder.