Other things to try also
Turn off your antivirus / security software to test directly from windows 7.
Some AV is overly aggressive and has been known to cause issues.
If thats your case then there are some simple steps to get around the problem.
DO NOT USE USB 3.0 ports/cables as these are also subject to problems.
If you have to then most people get around it by inserting a POWERED USB 2.0 hub between the computer and Arduino.
Double check that your USB cable is both DATA and CHARGE and simply not just a charge lead as a charge lead will not allow data to travel to the USB port of the Arduino.