issues with mixing and matching python packages

[John Schmitt]

I'm talking about the PythonLabs 2.0 versus ActiveState versus the installer
on Python.org.  I just glanced at the notes for the ActiveState installer
and it says:

- On Windows, ActivePython 2.0 cannot coexist with other Python 2.0
installations (for example, BeOpen's Python 2.0).  Whichever installer is
run last will determine what version of Python is used (the registry can
only point to one executable).  The ActivePython windows installer will warn
you if you try to install over an existing Python 2.0 installation. 

- You must ensure your PYTHONPATH is clean before installation. You should
ensure that your machine does not have PYTHONPATH, PYTHONHOME, or other
relevant variables set, and that your installation directories are clean. If
you have problems importing certain modules after installation, please check
these things before submitting a bug report. 

Does this mean that I can't simply change the installation I'm using by
simply changing some environment variables?  What's up?

What happens when I upgrade to 2.1?  Is that going to be a problem?

John