probably something to do with user access control, which seems to be the root cause of most of the vista problems i've seen so far. Go into user accounts in control panel and disable UAC for all users. see what happens

Can't say i have the same problem on mine though, Running Ultimate here, and my linux xbox connects from downstairs through smb/workgroup with no problems to stream music/movies etc whether the PC is logged in or not, then again, i turned UAC off within about 2 seconds of loading Vista onto my box