Thanks for the feedback.
I had already removed and re-installted WMP in the 'Turn features on or off', with no change. WMPNetworkSvc is running.
Hear's some further details however - on my main PC I have four user accounts. Most of them are little used as they are for the rest of the family, who now all have laptops. If I log on the the main PC in another user account, it works fine.
The other PC is my 'media' PC, with only one user account. So, I created a new user account - and on that account it works fine.
So i'm guessing it may be some other program that is installed, that I use but others don't, that is somehow screwing WMP up.
The Media PC does not have many programs on, as it's just for storing media and having a PC available on the TV. But there is one that is installed and used on both, (but not by other users on the main PC) and that's Cyberlink PowerDVD 10. Not gone as far as uninstalling this yet - may try actually running it on the new Media PC account and then see if that screws up the sharing.
If not I may just use the new user account and delete the old one. I'm not too worried about the main PC - its the media one I want it to work from. Also considering getting a Sony Tablet S to do 'the pushng'.