I am trying to run simple python code in atom using atom-runner package, but I am getting following error:

Unable to find command: python
Are you sure PATH is configured correctly?

How can I configure PATH. (path to my python is C:\Python34)
Right click the start menu, and select System. Then, hit "Advanced system settings" > "Environment Variables". Click on path, and hit edit. Select "New" and add the folder that your python executable is in. That should fix the problem. 
Your other option is to reinstall python and select "add PYTHON to PATH" as Carpetsmoker suggested.
Run python3 in atom with atom-runner