NetBIOS over TCP/IP is of course the same thing Microsoft incorrectly calls NETBEUI (there is actually a protocol called NETBEUI (IBM, 1985, NetBIOS Extended User Interface (NetBEUI)) that predates Microsoft's incorrect usage of the term by a couple years).
That aside, Microsoft has some issues in their "NETBEUI" and NETBIOS implementations that may be causing this issue. One I ran into is that when only binding NETBIOS in XP, it will still try to route certain traffic over the *UNINSTALLED* (heh) "NETBEUI" interface (interesting, that it binds it anyway, even though it shows it is not bound and has been removed).
What other bugs exist, I don't know - but I am wondering if there's some preference for using a "real" interface over the loopback when one is defined. If so, then you are out of luck. I do know that in early versions of XP, even certain TCP/IP traffic that should have been handled via the loopback, got passed to the "real" interface instead - perhaps it is more of the same, and never was fixed.