I use Win7 64-bit and had no problems with NSU.
I saw the 1.2 update in the App manager, i selected it and it prepared the installation. Then it told me to connect the phone to a PC and start the NSU. Because i didn't have it, i installed it and then connected the phone to my PC. After that the NSU downloaded the firmware and installed it. When it was done the phone restarted and i had 1.2.
No problems whatsoever. I can't believe all the problems people have been writing about over this.

My NSU is version 2.5.2en. The update only cleared the RootFS. Everything in /home and MyDocs remained intact.