Same situation here. Unable to install MTP to save my life. I used to have a Samsung Epic on Sprint then switched to an Atrix on AT&T and now to a S3 on VZW so I've not had issues with phones/drivers before and had even rooted the Atrix.
All I want to be able to do is add some ringtones and pics to my phone but it won't install MTP and the phone never shows up in my devices when plugged in. I installed the driver from Samsung already, even got an updated driver from Windows update for Samsung but no luck on getting MTP installed. When I plug the phone in, it says installing drivers and everything installs except MTP and the phone sits there.
FWIW, I'm running a home built Win 7 64bit machine that is about 2 years old. Never had any problems with any of the phones in the past either as far as installing drivers or accessing them via USB.
**UPDATE**
Just for kicks, I decided to install Kies on to my PC and play with it, once Kies installed the phone shows up now and MTP apparently works. I'd say give that a shot for anyone having issues, install Kies on the PC and see if that fixes the issue.
**New update**
Copied over some files, ejected the phone. Went back a little later to add some more stuff and MTP and drivers try to install again when I plug phone in and of course, you guessed it, MTP fails. A little frustrating.