Error in browsing lan when Win2000 srv is on!

Hi.

I'm having a strange issue. I've a little lan environment,
3 windows ME client + 1 windows 2000 srv.
All configured as workgroup.
When all computers are up, from win ME clients I cannot
browse the workgroup. But I can connect to a share
by typing the paath by hand, like \\computer\share

From the win2000 srv I can broswer the lan w/o problems.
The funny thing is that:
if I shutdown the win2000srv , I can broswe w/o probs
the net from the Me clients! As soon as the server is up,
the browsing stops again ;(

Any hint on where to check ?

Matteo