I've a similar issue on win7 64bit pro. The unit was working fine and then all of a sudden stopped. It shows up as a USB Serial Converter in the device manager.
I think its a driver issue, I use other FTDI usb to serial leads on the same machine.
When I plug my discovery into another machine running win 7 64bit pro is fires up and behaves as normal.
EDIT: I crawled around the back and plugged into an unused port, It's functioning but I need a usb extender/hub.
I just plugged it back into the original port and now it's working again.... Weird.