two wired PCs cannot see xbox360 and vice versa

Hello all,

I do not know it is because I reconfigured how I connect this two certain computers to my network.

So I have two computers that are connected to my router via a switch, and a couple of laptops connected wireless.
When I try to connect my Windows Media Center to my 360, it says that it cannot setup the extension, I tried following what Windows Online help had, but I still cannot figure it out.

I checked my router settings, and multicast is enabled (or so I think, I really do not know what that is), and I really do not know how to enable upnp.

My other computers that are connected to the network wirelessly, are able to see and communicate with the 360 just fine. Any help would be appreciated.

Thank you so much.