Today I was messing about with TVersity (trying to get streaming to iPods working), and got to a point where I deinstalled the software.
This then caused an issue with the Windows Media Player sharing setup with my Xboxes. Problem is in Vista there is no such thing as a deinstall/reinstall (which probably would have fixed it).
After a bit of searching I found this little gem…
TVersity on its uninstall had stopped the uPnP Device host service. I tried to restart it and that failed. Hmm. After looking at the dependencies of that service I found that it relied on the SSDP Discovery service. So I set that to auto and started it, set the uPnP Device host service to auto and restarted and behold, Media Sharing fires into life again.