I was having the same symptom, but your problem might have a different cause. Here's an excerpt from a very long and boring story about what worked for me:
...the VCP drivers from FTDI (CDM 2.08.24 WHQL Certified) are nice and signed. They install successfully, and Device manager shows a new entry "ATmega16U2" under a new class "Atmel USB Devices". No yellow exclamation mark, and its properties look good.
Maybe it'll work for you...
Cheers,
/dev