Vista Business reverts to default username at net logon

Attempting to log onto a LAN with my username and password, logon fails and
the next logon window shown has a bogus username assigned by Vista:
[Computername]\[Username], where [Computername] is the name of the local
computer I am using to log in from. My server will only answer to
[Username]. Network type is set to "Private".

How do I turn off this default?