Does Windows (some modern flavour) have an mDNS server installed and/or running by default? If so then how does one tell it, preferably dynamically, to start/stop advertising a
No, Microsoft doesn't directly support Multicast-DNS.
However, there appear to be several 3rd-party alternatives:
http://bens.me.uk/2013/multicast-dns-and-development-virtual-machines
http://en.wikipedia.org/wiki/Bonjour_%28software%29
http://www.icir.org/gregor/tools/autoconf-protocols.html
http://www.zeroconf.org/
... and ...
http://blogs.technet.com/b/networking/archive/2008/04/01/how-to-benefit-from-link-local-multicast-name-resolution.aspx