A friend of mine was also having a big problem connecting, and I'm not sure if this is the same problem you are having, but try this anyway.
This should work if you are trying to see your PC under the network menu on your box and can't find it (the tests say no PC connected).
If you go to the music player blade on the Dashboard (the blue one) and look around there, there should be something to do with connected PC's (I can't remember which menu it was under, and I'm away from my 360 at the moment - just look around under the Media blade and you should find something). Well, if you can find the screen I am talking about, it is likely that THIS screen will show your PC, allowing you to select it (I'm not sure how, but you should be able to figure it out), and then you can test the connection on the Settings blade.
Again, I don't know if this will help, but we spent a good few hours re-installing, fiddling with firewalls and testing everything before I randomly happened upon the Media blade and found the PC somewhere under that.
I hope this helps.