Xbox 360 Sharing Options
In order to view your files on your Xbox 360, go to My XBox > Video Library (or whichever library you wish to view files in). You should see your PC listed among the device sources, click this to see your shared content.
In the event that you can't see your PC listed, you'll need to follow the onscreen instructions provided by your Xbox 360 for configuring this. Once complete it's worth turning your Xbox off and back on again in order to complete the media sharing connection.
You may experience some difficulty at this stage. It is advisable at this point to disconnect your router from the internet and turn off the firewall on your PC, as well as any AV scanning software. With a restart of the services managing your Windows Media Player network sharing, any issues you've had should be resolved.
This can be done by going to your desktop, right-click My Computer and choose Manage. In the left pane expand Services and Applications, select Services and search for Windows Media Player Network Sharing Service and Universal Plug and Play Device Host, restarting each of these should sort the problem out.
With your PC firewall and AV disabled and your router disconnected from the internet, you may find reactivating each of these together will prevent further connections. If this happens switch each back on in turn and test your media sharing, leaving the internet connection until last.