You could try doing a repair by booting into the installation DVD, this will just make sure no system files are dodgey, though you should do a full backup of your files just in case.
But first, I would go to WMP12, go to library and highlight everything. Hit Delete, then make sure you choose Remove From Library only, you don't want to delete all your music. I would back-up all your music first just to be safe.
Then go to your music folder, highlight everything and drag it into the WMP12 library. This will recreate your library.
Then in WMP12 hit the Stream button (near the search box).
Then 'More Streaming Options'.
Hit Choose Homegroup and Sharing options. At this point I can only advise based on the screen I get.
Make sure you have Music, Videos checked, if that doesn't work try the share with all devices on my network.
Make sure you Save Changes.