client/server application problem

Hi all,

I'm running a server application on one machine (which runs Server 2003),
and have no problems when a client program on another machine accesses it.
However, when I run the client application on the same machine as the server
application, at the same time, it can't see the server application. I have
other set-ups of this program where both applications run on the same machine
with no worries.

Has anyone heard of a problem similar to this? I'm not sure where to look.
It seems the application can't see it's own computer on the network in this
instance. I had a look in the MS Knowledge base, but didn't find anything.

Thanks for your help.