Is Windows firewall turned on? I think Playon uses port 57331, but there is a guide at Playon’s site:
In your security software configuration settings, you must give full local and online network access to the PlayOn server executable. On normal installations, this can be found at C:Program FilesMediaMallMediaMallServer.exe. In our development labs, we use AVG and Windows Firewall. These consistently work fine for us, so we can highly recommend them, also because they don’t degrade PC performance as much as many other third-party security packages. If you’re having trouble configuring your security software to work with PlayOn, consult the forum. You may also want to take a look at Microsoft’s guide for setting up firewalls with Media Center Extenders, as it’s quite applicable to what’s needed for PlayOn as well.
I use Playon with my Live Plus’ with no problem. I am guessing your firewall is turned on.