My Arduino Uno with ATmega16U2 usb-serial chip is in a project.
So I use my Mega 2560 which has also the ATmega16u2 as usb-serial chip.
I removed the driver, and downloaded the zip-file of the Arduino IDE 1.6.0.
Here we go:
Using Windows 7 64-bit
In the Device Manager, "Other devices", there is it with exclamation mark.
Double click it. Select "Details", select "Hardware-id's".
I get : USB\VID_2341&PID_0010&REV_0001
That means the usb id is 2341:0010
Cancel properties, keep Device Manager open.
Unzip the downloaded file, folder "drivers", run dpinst_amd64.exe
Confirm to install.
The Device Manager automatically updates, and "Arduino Mega 2560 (COM4)" is shown. Without exclamation mark of course.
I don't know what happens when the zip-file is not unzipped. You should really unzip it (right-click on the downloaded file and select unpack or unzip or so).