Media Sharing stops working after reboot (Vista x64)

Hopefully someone has run into this issue before. I can get Media Sharing
(enabled through WMP11) working without any trouble. I can quickly verify
that my library is being shared by double-clicking on "Network" on my desktop
-- in the Explorer window that opens I see my local computer's Media icon.

However, whenever I reboot my computer, media sharing doesn't immediately
work -- connected computers can't see shared media files even though Media
Sharing is still enabled in WMP. What's more, the Media icon doesn't appear
in Network on my local computer.

The only way I've found to remedy this is to uncheck the "Share my media
to:" box in WMP (Library - Media Sharing), click OK, then go back in and
recheck the "Share my media to:" box and click OK. This allows networked
computers to see and access my media library and the Media icon reappears in
Network.

Either this is an undocumented bug (or at least I couldn't find anything on
it) or I'm missing something really easy. FWIW, my other two Vista machines
don't share this problem, but they're both running 32-bit Vista Home Premium
as opposed to x64 Vista Home Premium.

Please help!