Couple quick things....Not sure if this is your first sojurn in to networking, but, how are your IP's set up? Static, with all shares set in your HTPC? Media Center, or other, for the interface?
Are you in the Dashboard beta? I ask, because the previous update borked my network settings...I have my main ELITE set as static, and in the DMZ. I am in the beta, and after that last update, I lost all my PC connections and shares with my Xbox... some quick trobleshooting revealed that it had re-set me to Auto, for both the IP and DNS settings. My PC's are set to allow the Xbox, shared on the specific IP, MAc settings. they simply were doing as they were told, to filter/block all other inquiries.
"Once you learn good English...Can't nobody take it away!"
Xbox LIVE Gamertag: KEVIN G 40