Hello @ Ebuka,
Here is the following things you must try:
Running the command prompt as an administrator
As you're using Windows, installation should automatically edit the %PATH% variable. Therefore, I suspect you simply need to reboot your system after installing. You could also try restarting your program so they get the changed PATH.
If you install Node using the windows installer, there is nothing you have to do. It adds path to node and npm.
You can also use Windows setx command for changing system environment variables. No reboot is required. Just logout/login. Or just open a new cmd window, if you want to see the changing there.
setx PATH "%PATH%;C:\Program Files\nodejs"
Hope it helps!!