I've been having some problems with MTP and I've been searching for days and finally found a solution!
My problem was that MTP was showing a yellow exclamation in device manager and an error "Code 31"
After installing, uninstalling and re-installing dozens of drivers I found this solution:
1. With phone unplugged open cmd (Run as Administrator)
2.Type this command:
net localgroup Administrators "local service" /add
Check this thread for more info - all credits here: http://forum.xda-developers.com/show....php?t=1115366