Add .PY to PATHEXT as noted before
Then do:
assoc .py=Python.File
ftype Python.File=c:\Python27\python.exe "%1" %*
Adding python to the path isn’t necessary to execute the script in a command prompt or double clicking in Explorer, only if you want to start an interactive python session or running the script with python yourscript.py
See http://docs.python.org/2/using/windows.html for more details.