Neighborhood Network Update

When double clicking Neighborhood Network in Win 95/98, or My Network Places in Win2000, a list of computers on the network is displayed. But this list is not always a accurate listing of computers currently present. Some computers could have loggedoff or logged on since this list was last generated.
What is the time interval or event that generates the need to acquire a listing of computers on the network? And how can you instruct windows to update the displayed listing immediately?