在CMD“python”启动python 3.3，“py”启动python 2.7，我如何改变它?

Brand new Python, just getting things set up and installed before I start messing around with things. My understanding is that there are some notable differences/incompatibilities between Python 2.7 and Python 3.3, though both versions are well used, so I thought it best to install Brand new Python, just getting things set up an