This is a strange one. I have three pcs, all windows xp pro, on same network as DSM-520. All three have WMP 11, same version. In WMP, all are setup identically for sharring.
For simplicity, I will call the pcs as pc1, pc2 and pc3. In media player, I see all three. When I click on pc3, message says Registering... Connecting, you can now play protected windows media files.
If I select pc1 or pc2, I get this message, Registering the device, connecting..... Registration failed. You CANNOT play protected windows media files.
What is missing or different in pc1 and pc2. Have tried turning firewall on and off, no change.