Is it possible to run two versions of Python side-by-side?
OK, I figured out the answer to my own question, partly with the help of Nicholas Knight who pointed out that you just install different Python version to different Python directories.  I was left scratching my head on how to get Google App Engine to use Python 2.5 (the required version) instead of Python 2.6.  This is the answer:
1) Install Python 2.5.
2) Install Python 2.6 (or a more recent version), afterwards.  This will be the system default.
3) Install the Google App Engine SDK.
4) Launch, "Google App Engine Launcher" from the Start Menu
5) Click Edit > Preferences, and enter the path to the pythonw.exe executable.  Usually c:\Python25\pythonw.exe
I've been learning Python for a couple of weeks, and although I've been successfully develop apps for Google App Engine with Python 2.6.5, it specifically requires Python 2.5.
Being mindful of compatibility issues when uploading apps (it's a situation I'd rather avoid while learning Python), I wonder if it's possible to have 2.5 and 2.6.5 installed on the same machine.  Ideally I'd like to use 2.6.5 as the default, and configure GAE to somehow use 2.5.