For Windows 10/8/7:

1. Open System Properties (Right click Computer in the start menu, or use the keyboard shortcut Win+Pause)
2. Click Advanced system settings in the sidebar.
3. Click Environment Variables...
4. Select PATH in the System variables section
5. Click Edit

6. Add Python's path to the end of the list (the paths are separated by semicolons). For example:

   C:\Windows;C:\Windows\System32;C:\Python27
   

For Windows XP:

1. Open System Properties (Type it in the start menu, or use the keyboard shortcut Win+Pause)
2. Switch to the Advanced tab
3. Click Environment Variables...
4. Select PATH in the System variables section
5. Click Edit

6. Add Python's path to the end of the list (the paths are separated by semicolons). For example:

   C:\Windows;C:\Windows\System32;C:\Python27
   

7. Test on a new terminal window or if using an integrated terminal within a text editor, close and restart your editor or the changes won't be applied.

You know what has worked for me really well on windows.

My Computer > Properties > Advanced System Settings > Environment Variables >

Just add the path as C:\Python27 (or wherever you installed python)

OR

Then under system variables I create a new Variable called PythonPath. In this variable I have C:\Python27\Lib;C:\Python27\DLLs;C:\Python27\Lib\lib-tk;C:\other-folders-on-the-path

This is the best way that has worked for me which I hadn't found in any of the docs offered.

EDIT: For those who are not able to get it, Please add

C:\Python27;

along with it. Else it will never work.