The mystery of Samba and the Master Browser

In a Windoze workgroup you need one of the machines to be a "master browser" that keeps a record of the machines on the network and allows each machine to access this information in order to browse the "Network Neighborhood". The way this is done is to "rank" each machine according to its operating system (i.e. Windows XP beats Windows 98 and Windows 2003 Server beats Windows XP etc). If there is no current browser, an "election" is held and the machine with the highest ranking wins. The "Browser" service in Windoze handles this.
I have a Samba file server running on Ubuntu and it is set up to be the master browser as it has the capacity to handle more tasks and is almost always available. The problem is that in a mixed OS workgroup (Ubuntu/Win XP/Win 2000) this is not working and when any of the users attempts to browse the "Neighborhood", they receive a message that says they do not have rights ... Annoying! Running Wireshark revealed that the "__MSBROWSE__<01>" key is not set on the Samba machine or any of the others.

I have spent a lot of time Google-ing the issue and it seems to be a common problem but not many good ideas are around for resolving it! The usual "make sure your Samba machine is set up as domain/local/preferred master in smb.conf" and "make sure your OS level is high to win an election" don't seem to sort it out!

I have tried various options in Samba and the following seems to be a winning combination:

1. Allow guest access on at least one share.
2. A OS level of 65 seems ok but wont beat a MS Server Domain controller if present which is good (DC must be master browser)
3. Make sure "global" is chosen as the default service.

I'm not sure exactly which one is the magic setting but I am able to browse the network again...