Since node prefix is not in the PATH ENV variable , any of the globally installed modules are not getting recognized.
Please try this.
Open cmd prompt
npm config get prefix
append the resulting path to PATH env variable.
Now you should be able to run nodemon from any location.
This is what i have done on my local machine
C:\>npm config get prefix C:\Users\username\AppData\Roaming\npm C:\>set PATH=%PATH%;C:\Users\username\AppData\Roaming\npm; C:\>nodemon 31 Jul 22:30:29 - [nodemon] v0.7.8 31 Jul 22:30:29 - [nodemon] to restart at any time, enter `rs` 31 Jul 22:30:29 - [nodemon] watching: C:\ 31 Jul 22:30:29 - [nodemon] starting `node ` ^CTerminate batch job (Y/N)? Y