Open the Start Menu, and type "Environment". Select the option labeled Edit the system environment variables. A window will open, select Environment Variables in the bottom right. The top portion of the window has a list of variables for the user currently logged in. Select the variable titled Path and then Edit…. Click New to add the Python installation path.
Now when you want to run a Python script in command prompt, you can just type
Python instead of the full path.
Trying to run iPython inside of a virtualenv virtual environment will likely result in this error: WARNING: Attempting to work in a virtualenv.