I'm not going to go into the details of how you set up Windows Media Player in Windows 7, it's pretty self explanatory, streaming across PC's on the same network works well once you have a homegroup set up... until you get to the point when you want to stream across the Internet.
I was able to stream when in the same network, my office PC was in the same homegroup as the one at home and I'd set up my authorised users but still it wouldn't work...
After much head scratching I decided to give Microsoft a call as they must have experienced the issue before.
Initlaly they said it wouldn't work because my laptop (the machine in the office which was the client) would not be able to play music from my home PC as it was a member of a domain.
After reading the message out loud to the engineed a few times he accepted that it did indeed state that a Domain PC can't be the media server, but can be the Client.
Then we had the confusion because my home PC was Windows 7 Home Premium and the work one was Windows 7 Professional.
Finally I get escallated to someone else who then asked me the all important question:
Have you configured your firewall?
Apparently the firewall should be configured automatically by UPnP, but as a lot of routers are unreliable with UPnP it's a good idea to enter the port forwarding directly.
If you click on Stream - Allow Internet access to home media... you get the option to Diagnose Connections.
You need to set your home router up to forward TCP/44687 to TCP/10245 to your home PC.
I hope this helps you out.. if it does, please click on the ads on this page to show your appreciation.