If the Windows Firewall service is disabled, under Windows XP, then the computer will not be able to become a browse master. This in turn means that it will not be able to advertise the workgroup it is a member of.
I do not know how or why the Windows Firewall service affects this. All I know is that it does, and it shouldn't.
I have just spent several hours going round in circles trying to fix this.
(apparently this is a known bug (KB 889320), but you can only get the fix by phoning Microsoft or waiting for XP SP3)