ntFaq to the rescue:

http://www.ntfaq.com/Search/SearchResults.cfm?SearchString=master%20browser

http://www.windowsitlibrary.com/Content/155/05/2.html

>I received the following message in my NT 4 Server Event Viewer:
>
>The master browser has received a server announcement from the computer
>DELL_WKS that believes that it is the master browser for the domain on
>transport NetBT_CpqNF31. The master browser is stopping or an election is
>being forced.
>
>Since this message appeared, the users were unable to log-in to the
>Server.
>I guessed that it is the DELL_WKS (loaded with NT 4 Workstation) is
>giving the problem. How can I default the main NT Server to be the
>Master Browser, rather that automatically granting any powerful workstation
>to be the Master Browser? Pls advise, thank you.