npm error on every command: EEXIST: file already exists, mkdir 'c:\users\user\appdata\Roaming\npm'

Try reinstalling node and npm for the non-admin account. If that's not an option, you can use the portable version:

Download from http://nodejs.org/dist/
Extract the archive.
Open cmd and cd to the extracted folder.
Run nodevars.bat.

It will set PATH and other things so you can now use node from this folder.

In my case remove white space from folder name.

I also had this problem. Just delete npm folder, it will be created again. This problem is due to user privileges.