Wierd Network problem

I recently purchased a Zyxel wireless router/access point,
and equipped two of my three home computers with a zyxel
PCI card. The third computer is hard wired to the router.

One of the PCI-equipped computers is a Win 98 SE machine,
as is the hard-wired one. The third computer is a new XP
machine.

Initially, I had the two 98SE machines set up, and sharing
perfectly. I set up WEP security, and also set up some
file sharing shortcuts between the two computers. Then, I
tried to implement MAC address filtering for added
security. During this effort, somehow something happened
to the WIFI machine, and so I backed out of the MAC
security attempts I was doing. In the process of trying
to figure out what was wrong, I reinstalled drivers, and
did quite a bit of trouble shooting, but to no avail. I
have gotten back to the following situation:

I can see shared resources on the WIFI computer from the
wired computer by use of the existing shortcuts. However,
the WIFI computer does not show on Network Neighborhood.
The WIFI computer, however, cannot see the wired computer,
network resources that are on the wired computer cannot be
run from the WIFI computer, but the WIFI computer can
access the internet through the access point.

Since then, I installed and successfully set up file
sharing on the XP machine (the third machine) which can
see and share resources with the wired computer, and the
wired computer can see and share resources with the XP
machine. However, the XP machine cannot see the WIFI 98SE
machine.

What could be wrong with the WIFI machine running 98SE?
Zyxel said that it could be something wrong with the
NetBIOS, but I really have no clue. It seems that the
card is working, there is no problem finding the internet,
but it cannot find the other computers on the LAN.
Workgroups are all spelled the same, with legal file
names. Any ideas?

Thanks in advance!