Showing hidden devices on Windows

Courtesy of the good people of the libusb-devel mailing list:

1. Set a System (not user) environment variable called:
   DevMgr_Show_NonPresent_Devices with value 1
   
2. Tell device mgr (thru menus) to show hidden devices

See http://www.ftdichip.com/Documents/AppNotes/AN_107_AdvancedDriverOptions_AN_000073.pdf

This trick seems to work fine on Windows 7 as well.