The above steps shuld get you into the right direction.
I have had this happen to me a few times, each time I have to reset the master browser.
Be warned sometimes it was software I think the only one I remember is MS Money 2004/5 which broke my internal workgroup.
Here is some info on the error from MS:
http://support.microsoft.com/default...b;en-us;318030

What I had to do was stop the browser service on all the machines and then reboot them all one by one, I took my main desktop up first and then all the others one at a time.

HTH