I tried running npm install -g angular-cli
I also tried adding it to the Enviorment Variables under PATH: (C:\\Users\\Administrator\\AppData\\Roaming\\npm\\node
I had the same issue on Windows7. I resolved it setting correct path.
First find ng.cmd file on your System. It will usually at:
E:\Users\\AppData\Roaming\npm
Set PATH
to this location.
Close existing command window and open new one
Type
ng version
Also remember to install angular with -g command.
npm install -g @angular/cli