I spent 5 hours messing with drivers, setting the windows registry to show hidden devices in Device Manager (there were a lot of old COM ports!), reinstalled drivers, user the add hardware wizard from the Run start menu prompt, tried reprogramming the bootloader on the 8U2 with another working arduino board.... all to no avail.
Hitting the 8U2 with a hot air gun for 10 or 15 seconds did the trick! 5 hours to learn I need to get a loupe or something to inspect solder joints!