Is there a Bonjour SDK for Windows?
Have a look at Apple's bonjour SDK page. They provide an SDK for Windows there. I think it's written in C.
This page describes how to develop for Bonjour for Windows. It also gives a link to the download page for the Windows API, including headers and libraries for C and Java.
Note that Bonjour is only for notifying other devices that a service is present on the network, it doesn't handle the actual file transfer.
There is a Java implementation called Rendezvous Proxy that broadcasts bonjour notifications, may be all you need on your server?
Web page is here: http://ileech.sf.net/RendezvousProxy/
Don't think the source is open though.