I am having the exsact same issue, I know its not a bad usb cable or bad usb port as a friends computer communicates just fine with the phone and loads the drivers. I replaced the phone due to bad usb port. all the drivers were previously loaded and working fine. now the new phone always comes up unkown device even after manually installing all the needed drivers.
Ive also tried reinstalling windows 7 and installing windows xp and still nothing.