Using multiple versions of Python

Google search results have returned a few useful resources that answer your problem.

Python Docs

The Python Documentation (http://docs.python.org/3.3/using/windows.html#python-launcher-for-windows) gives a quick overview for running multiple versions on the same machine.

The first option would be to include your python version in the file you wish to execute using something along the lines of

#! python
Your code here

To execute in Python 2, or

#! python3
Your code here

For running the code in your Python 3 version. Then you would simply use "python yourscript.py" and the python version would be specified by the Python script.

StackExchange Sites

There are multiple other questions which may address the problem you are facing:
How to install both Python 2.x and Python 3.x in Windows 7

Or for Ubuntu 13: Ubuntu 13.04 Install and running Python 3 at the same time than Python 2.7.x

Or for Mac using Homebrew: How can I use Homebrew to install both Python 2 and 3 on Mac?
And a video reference for Mac without Homebrew: http://www.youtube.com/watch?v=c9LlK2iu7OA

You will get issue if multipler version of python is set

File "F:\PYTHON33\LIB\site.py", line 173 file=sys.stderr) ^ SyntaxError: invalid syntax

To fix this issue remove previous version python set in system environment variable