Be patient. iTunes is extremely slow on Windows, and 64 bit windows version is still a bit beta. I have a very similar set up and it shares the libraries between 3 PCs (win 7 64bit). As I have a very large library, iTunes can take several minutes to load it up on the older PC, and that's not a function of the speed of the network.
!. Check (Edit, Preferences, Sharing) that the library is shared inside preferences. The Home Sharing check box is optional.
Check that each version of iTunes on each computer is authorized to the same apple account. I'm not sure if this is absolutely necessary but that could be why you are having trouble (I have one account and so does my hubby, but our itunes has both accounts registered).
Check that the "other" pc can "see" your main iTunes library outside of iTunes - network security, file sharing may not permit the other pc access to the iTunes folder. I don't use "home group" windows sharing and iTunes can't share folders that windows locks down.
As to the iPhone, I use an app to "see" the iTunes library. Otherwise the phone only plays what was sync'd to it. The remote app works just fine on the PC that is playing the file. I hope this helps.