Norton Internet Security 2010 and xbox 360 Media sharing Issues

I have two PCs, both are running Norton 2010 and I use WMP 11 to stream media to my Xbox.

 

Now my question is, one PC shows up always under SHARED media for my xbox and the other PC needs to have Firewall turned off first.

 

 

Why is that?  As far as I know all my settings are identical on both pcs.

 

so why does one pc need to have the firewall disabled and the other not?