Basic Definitions - Help Please!

I have a problem that I really don't know how to define. See, I have a home network that consists of 3 computers. One is connected directly to the router via an ethernet cable and the two others are wirelessly connected. All 3 can access the internet, however, I am having trouble "seeing" the new wireless computer in "My Network Places." I cannot access the computer from the others, nor can I access the others from it. The other two are connected just fine, however.

I am having a hard time defining if this is a network setup problem, a firewall problem, a router problem, etc. Whenever I start the network wizard, it asks me how the computers are connected to the net. Since each is connected to the router, do I say that all are connected individually to a network hub? Or does the router function as a gateway?

Also, will the D-Link router act as a sufficient firewall? If so, can I turn off the software firewalls? What's odd about this whole thing is that the software firewalls were on and the first two computers were sharing their files just fine... that really confused me!

Any help on these basics would be greatly appreciated!