SQL 2005 Express on SBS2008 connection issues from Vista x32 SP2 machines

I have a VB6.0 application running at a client's site on Vista SP2. When attempting to connect to a SQL 2005 Express database on a named instance ([edit]running in Mixed Mode – not Windows Authentication Only), on a SBS2008 server, from THREE OF THE FOUR Vista workstations in the office I receive the following errors:

"SQL Server does not exist or access denied"

(Using either the sqloledb provider or SQL-DMO)

Of course, the fourth Vista Workstation connects without a problem.

I've tried;

1) Creating a UDL (data link) file in order to "triple" check my connect strings and even when attempting to connect here (selecting the Microsoft OLE DB Provider) I receive the same error when it attempt to refresh the list of available databases

2) I have checked firewall exceptions on the server and even tried the tests, with the firewall turned off.

3) I have added outbound exceptions for my application to the firewall on the Vista machines.

4) I have installed the SQL2005 Backwards compatibility objects.

5) I have installed SQL Server Management Studio on one of the offending Vista machines and this errors in the same way.

6) I have also simulated the test environment in our offices on virtual machines and of course, no problems…

I guess my question is, how to I find out what is different about the one Vista PC that does connect, as opposed to the three that do not?

(Update) Also:

A Virtual Server has been added to the SBS 2008 Server, running SBS 2003 and all those offending Vista Workstations connect without a hitch??