I have Verizon FIOS. The ActionTec router supplied by Verizon for use with FIOS does not allow any users wired or wireless to see each other. To solve this problem, I connected a Netger Wireless N router to the ActionTec router. The computers upstairs are wired, and the computers downstairs are wireless, but they can all see each other.
Verizon does this as a security feature, so that if one computer in your home network gets infected, it can't reach out and infect the others. This can be overridden in a standard ActionTec router, but not in the ones with the Verizon firmware.
Do you have Verizon FIOS? If so you will have to get another router in order to set up a home network. If you don't have Verizon FIOS, log on to your router, and see if it is set to not allow users to see each other, and if you can reconfigure it.