I think somehow that once Windows screws things up its hard to recover.
Windows now completely hangs while looking for drivers for this, so I suspect its a bug in Windows
W7 often takes a very long time looking for drivers, but in this case it really does hang, as I can't close the dialog without killing it in the task manager.
Because W7 takes so long to automatically look for drivers, even for drivers its already got installed, I often just hit cancel if its looking for drivers because I've plugged the same device into a different USB port. It usually then realises that its really got a local copy of the driver and installs instantly.
With the Maple Serial and Maple DFU drivers - whatever libwdi does, seems to prevent Windows from looking online for the driver.
If you build onto a different VID/PID I can try it again, and perhaps be more patient, however I suspect most people would give up long before I do, as we have had numerous USB issues reported.
I now understand why Arduino.cc mainly for separate dedicated USB to serial ics on their boards, as they are far more reliable than using bootloaders and you don't need to worry about your own drivers.
STM32duino bootloader aka Maple bootloader