I was having the same problem with my Zen Vision:M under Win7 Ultimate 64-bit.
No matter what I tried from deleting the usb root hubs and reinstalling drivers, nothing would get the mtp usb device driver to load.
I found that the mtp usb device driver will load correctly as long as you are plugging the USB cable into a direct port and not running it through a USB hub. At first I thought it might be due to an incorrect USB voltage, but the hub I'm using is a powered Belkin hub. Very strange!
So give that a shot -- run the USB straight to a port out the back of your computer. Don't use a USB hub or ports off your monitor since that is actually a hub as well.
Good luck!