I'm running a clean install of Windows 7 (about a week old). The Moment is mounting the SD card without any problems at all. But, if you turn on the USB Debugging under Settings-->Applications-->Development, then you can't mount the SD card because the driver for USB Debugging isn't found by windows. Just turning it back off will allow you to mount it.
If that's not the problem, then perhaps because I had a Hero mounting with windows previously, perhaps it did something that enabled the Moment to work.