There is some sort of trust or connection established between the Workgroup you computer is in, and the domain you wish to join. I would look in AD Domains in Trusts for references to the Workgroup your machine is in. Also look in the registry on the machine in question for refernces to the domain you wish to join.
Another thing to try would be changing the name of the Workgroup your machine is in to another non-existent one, and then joining the domain.
-gbiello