When you run the Windows Command Prompt, and type in python, it starts the Python interpreter.
Typing it again tries to interpret python as a variable, which doesn't exist and thus won't work:
Microsoft Windows [Version 6.1.7601]
Copyright (c) 2009 Microsoft Corporation. All rights reserved.
C:\Users\USER>python
Python 2.7.5 (default, May 15 2013, 22:43:36) [MSC v.1500 32 bit (Intel)] on win32
Type "help", "copyright", "credits" or "license" for more information.
>>> python
Traceback (most recent call last):
File "<stdin>", line 1, in <module>
NameError: name 'python' is not defined
>>> print("interpreter has started")
interpreter has started
>>> quit() # leave the interpreter, and go back to the command line
C:\Users\USER>
If you're not doing this from the command line, and instead running the Python interpreter (python.exe or IDLE's shell) directly, you are not in the Windows Command Line, and python is interpreted as a variable, which you have not defined.