add
;C:\Program Files\nodejs\
To the end of your Path variable on the "User variable" section of the Environment Variables on the System Properties.
After that, reopen your command prompt and type
npm This should work.

Just add:
;C:\Program Files\nodejs\
To the end of your **Path** variable on the **"User variable"** section of the Environment Variables on the System Properties.
After that, reopen your command prompt and type
npm
This should work.