python from any command line?

[waltbrad]

Hi folks. I'm learning Python from the Mark Lutz Book, Programming
Python 3rd edition.

He seems to be able to invoke the Python interpreter from any command
line prompt.

C:\temp>python

C:\PP3rdEd\examples>python

C:\PP3rdEd\Examples\PP3E\System>cd

Whereas I am only able to invoke it when the command line is pointing
to the directory where the executable resides. Currently:

C:\Python25

Is there a way to set this so I can also invoke it from any command
line prompt?

Thankyou very much.