New computer has stolen control of network

Tuesday Eve

Mixed OS network: 2 - XP pro( sp1, sp2), 2 - 98se.

The network has been working fine for years.

My main computer is not using Simple File Sharing.

I added a new XP pro sp2 box, using Simple File sharing.

All works fine.

BUT, when this new computer is powered-OFF the local network is not available, including printers.

AND when the new computer is ON the Computer Browser Service stops on my main computer.

I am more than just annoyed.

Both computers are running NAV 2005.

When the new computer was first connected I had disabled the Windows Firewall Service (among others I do not want running for no
reason). But this stopped the network as well.

Help please.