Had this problem this past week.
You need to uninstall all Samsung software and drivers completely. Do this the normal way through Control Panel.
After that, you need to go into your registry and delete all Samsung folders.
You open up Regedit in Windows
Then go to HKEY CURRENT USER, Expand that (+), expand Software
and delete the Samsung folder
Do the same for HKEY LOCAL MACHINE
When you simply uninstall Samsung stuff, it leaves a bunch of junk in the registry of the computer even after uninstall.
Reboot the computer with no phone attached.
Download the Samsung update from Sprint
http://www6.sprint.com/downloads/samsung/executables/SamsungMoment_Update.exe
Install the updater. After its installed, right click the SWUpgrade and run as administrator....Follow all on screen instructions, and you should be good.
If you do not need the updater, then you can skip the software install and just download the drivers for the phone from the SDX forum. Just do a search.
But I suggest you do it this way, it was the only thing that worked for me...
Running Windows 7 Ultimate 64
Hope this helps
Steve