Can't turn on media sharing

Windows Vista Ultimate

I can't turn on Media Sharing under the Network and Sharing Center. I open
the Media Sharing window, select 'Share my media' and click on OK. Nothing
happens, the window just stays up, no error message but also no sharing.
I can't find any error message in the Event Log.
The computer is not on a domain controlled network, just a workgroup.
Any ideas? I don't even know where to start since there is no error message,
the window just sits there.