A user on xda-developers.com has created a tool to enable and disable the USB Mass Storage feature on your Windows 7 device, with an easy to use interface, by just using the Zune software, and .Net 2.0.
“According to the developer’s website, ultimate-networx.net (loosely translated from German), you will need to run the tool as an administrator for it to work correctly. Once it is enabled or disabled, you will need to disconnect the device and then plug it back in.
Upon checking drives listed in Computer (in Windows 7), you will see a device listed with the phone model, and files can be dragged and dropped to/from the device at will.”
You can download the tool from the XDA-developers.