Ah, Apple. Such incredibly desirable products! Yet such poor understanding of the world of the PC. It would be nice for iTunes to be aware when problems like this happen...oh well.
If you have Windows 7 and are trying to share your iTunes collection on the network, you'll probably have to open up TCP and UDP ports on your firewall to get it to work. Apple has instructions on how to do this with the built-in Win7 firewall but since so many use 3rd party firewalls, your process may vary. Bottom line is that TCP 3689 and UDP 5353 need to be cleared as both internal and external ports for all applications.
Now, say you'd like to use your iPhone or iPod Touch as an iTunes remote? Well, many modern wireless base stations come configured to have the "Filter Multicast" option set to off by default. This is a problem, as iTunes Remote requires this functionality. What is multicast filtering?
Multicasting allows for multiple transmissions to specific recipients at the same time. If multicasting is permitted, then the Router will allow IP multicast packets to be forwarded to the appropriate computers. Select Enabled to filter multicasting, or Disabled to disable this feature.
Yeah, I don't understand either. But you gotta turn it on, or Remote won't work. (Oddly, it works on Macs without this setting.)