DCOM callbacks and "Simple file sharing" (http://www.sandon.it/node/42)
In the past days I hit another DCOM issue: callbacks do not work if "Simple file sharing" is enabled (Settings → Control Panel → Folder Options → View). That's because the full name of that option is "Simple file sharing and ForceGuest (http://www.microsoft.com/technet/security/advisory/906574.mspx)". When enabled, Windows will ignore incoming calls credentials and will authenticate everybody as "guests". And of course a DCOM callback won't work properly this way unless permissions allow such kind of access (dangerous!).
"Simple file sharing" is enabled by default in WIndows "home" editions, and usually in Professional/Business/Ultimate editions when not joined to a domain.