BOAC,
"The system detected that network adapter Compaq NC6134 Gigabit NIC (NB Not MINE) was connected to the network, and has initiated normal operation over the network adapter. A single TCPIP 4201 event is typical after you restart the computer or after you disable or enable the network adapter."
MS are just lazy in their entry -
Compaq NC6134 Gigabit NIC - should simply be
Your Network adapter. There is nothing mysterious about that - there are loads of 4201 in my System Event log.
The Master Browser hasn't necessarily stopped - I suspect that it just can't communicate because your network card is yo-yoing.
SD