Solution 3: Update the device driver for your Windows Phone 7
To start, connect your phone to your computer using the USB cord that came with the device, and then follow these steps on your computer:
Click Start, right-click Computer and then click Properties.
If you’re using Windows 8: From the Start screen start typing “computer”. When it appears on your screen, right-click or swipe the Computer icon and then click or tap Properties.
Click or tap Device Manager, located in the left column.
A window will appear with the name of your computer and a list of files. If you don't see the files, click the arrow next to the name of your computer to see the list. The name of your Windows Phone should be listed under Portable Devices. If Windows Phone was incorrectly installed, it may be listed with a yellow exclamation mark. Or, it may be listed under Other Devices as a USB device or as an unknown device.
Right-click or swipe the name of your phone, and then click Uninstall.
Select Delete the driver software for this device, and then click or tap OK.
Return to the Device Manager window, click or tap the Action menu and then click or tap Scan for hardware changes.
Open Portable Devices, double-click or swipe the name of your Windows Phone and then click or tap the Driver tab. Or, if you don't see your phone name, click or tap Unknown device listed under Other Devices and then click or tap the Driver tab.
Click or tap Update Driver, and then click or tap Browse my computer for driver software.
Click or tap Browse, locate the Program Files/Zune/Drivers folder on your computer and then click or tap OK.
Click or tap Next, and if you're prompted, click or tap Install. You might only be given a Close option.
After the driver installation is complete, closethe open windows until you exit the Device Manager and then restart your computer if you’re prompted.