Right click the active sync icon in your computers taskbar down by the clock and go to connection settings. Make sure "Allow USB Connections" is selected.
If that doesn't solve the issue, on your phone, go to start menu > settings > connections > USB to PC, and then make sure you deselect the "Advanced Network Functionality" or whatever it's called. Sometimes that helps.
If neither of these work, reply back and we'll go from there
