Just fixed this for myself on Windows 7, seems like the 'decade long windows expert' here is not of much help.
Go to Device Manager > find the Unknown Device(which is your Xbox 360 controller connector) > right click on it > click Update Driver Software > Browse my computer for driver software > Let me pick from a list of device drivers on my computer > Pick Microsoft Common Controller for Windows Class and finish the installation wizard. Enjoy your 360 controller again!