The first generation of Windows Phone allowed the easy transfer of files to and from a computer, but things tends to become more complicated since Windows Phone 7. For the transfer of data, a file manager such as "Zune" is normally required. However there's a registry hack to turn Windows Phone into a USB key.
On your PC:
Click on Start > Run.
Go to HKEY_LOCAL_MACHINE > SYSTEM > CurrentControlSet > Enum > USB.
Right -click the "USB" folder and select "Search".
In the search box, type "PortableDeviceNameSpace" and navigate to the "Device Parameters" folder.
In the right column , double-click "EnableLegacySupport" and change the value from "0" to "1".
Double-click "PortableDeviceNameSpaceExcludeFromShell" and change the value from "1" to "0".
Finally, double-click "ShowInShell" and change the value from "0" to "1".
Close registry editor .
You can now connect your Windows Phone 7 device to your computer using the appropriate USB cable. If the file management software (Zune) starts automatically, close it and open Windows Explorer. Expand the "Computer" section to access the memory of your smartphone. Files can be transferred via simple drag and drop.