Please help! Can't seem to see the new computer from the old one

I have an established Linksys network. It previously had
only 4 devices (3 computers and a snap server). I have
added a new computer. One of the older computers is unable
to see the new one and the new one is unable to see the
old one.

The configuration is as follows: Cable/DSL router, 4-port
hub, wireless hub. Everybody has access to the internet
and the printer. The snap server is attached to the
router. Computer "A", running Win 98SE, has the printer
attached and can see all devices on the network. It is
attached to the router. Computer "B", running Win98 SE, is
attached to the hub. We don't care what it can see, it is
setup without a monitor. It's only purpose is to share
it's disk drives with the rest of the network.
Computer "C", running Win 98SE, is a laptop with a
wireless PCMCIA card. It can see everything except
Computer "D". Computer "D" is the new one running Win XP
home editiion. It is using the wireless hub. It can see
everything except the snap server and Computer "C". We
need it to see the snap server. I don't really need "D" to
see "C", but I REALLY want "C" to see "D" for file sharing
purposes.

I've gone to practicallynetworked.com, annoyances.org,
careyholzman.com, and Microsoft and just seem to get more
confused.

Linksys will only support internet access through their
technical support. File sharing is not supported. But I
suspect it has something to do with the way I have the
Linksys devices "daisy-chained" together. The only
computers that are having problems are both attached to
the wireless hub.

I apologize if this has been asked and answered before.
Please just point me in the right direction. Any and all
help is GREATLY appreciated.

TIA
Nadine H.