FreeBT is an open source Windows Bluetooth stack implemented by Antony C. Roberts http://www.freebt.net.
This is original FreeBT implementation with a number of my modifications in user space code. My modifications were initially released under GNU GPL v3 license for the Bluetooth for Microsoft Device Emulator project and re-licensed here under the FreeBT license.
You can still download compiled FreeBT USB driver from http://code.google.com/p/bthmsdevemul/downloads/list.