Back when i was doing usb driver develoment and XP was new, XP use to have a inf folder inside of windows\system\drivers\inf\ (or something similar) that kept the INFs for all USB devices. I'm not sure where 7 would store the same thing.
Maybe see if there is a inf file containing the word "Arduino" or "Uno" lingering around. That should make device manager think it is a new device being plugged in.