Added router, now "network is busy" error

Hi all,

Four computers, all running Win98. After years of problems with
WinGate, I threw up my hands and bought a router yesterday.

Now we have internet on all four computers, but on one computer -- the
former WinGate server (named GROUCHO) -- it can see the other three
computers, but no other computer can access it ("the network is
busy.") The other computers (CHICO, HARPO AND ZEPPO) can see "Groucho"
in Network Neighborhood, but when they try to connect they get "the
network is busy" or "permanent connection not available" errors.

I suspect this has something to do with the fact that the OLD
internal-network card has been removed and the "modem" network card is
now connected to the new router.

Am I missing a protocol or something? GROUCHO seems to have the same
protocols and adapters installed as the other computers, but I keep
getting this recurring error. GROUCHO can print across the network to
a network printer on HARPO, but HARPO can't access GROUCHO. Etc.

Thanks for any help!

Am I in big trouble here?