Thanks!! This finally solved my problems (I have a wireless router that came with my Verizon FIOS) as my firewall, antivirus settings, and other settings were not the problem or were in accordance with other threads on this matter. I am now finally able to concurrently access my 4 Airport Express Devices (setup wirelessly and hooked up to home theater system, portable speakers, & stereo system in different rooms) via Itunes from my two networked (ethernet cable) PCs (One running Vista and the other running Windows 7) and my Daughter's Mac (wireless), plus use my IPOD Touches as remotes for two of the PCs (Of course, they are accessing the same wireless network).
In addition to one of the IPOD touches being used as a remote for a PC playing Itunes, I can concurrently use it to play the Music App (for music actually resident on the IPOD) using Airply and directing to the 4th Airport Express.
Each Itunes program on each computer or IPOD Touch is playing it's own Library and different songs. Of course, although I'm doing this concurrently, all deivces are directed to a different Airport Express device. This is all being done using one wireless router with all wired devices on the same segment. Although I have noticed that I have to start one PC first before the other before this all works (don't know why), the MAC never gives a problem.
Note related to this issue, it would be great if I could run multiple instances of iTunes on the same computer using different Music Libraries and directing it to different Airport Express devices in addition to the above.