Separate Windows versions of Python

[OldGrantonian]

I use Windows Vista Home Premium. I have Python 2.6 currently
installed. I'm not a techy.

I want to use "virtualenv". When I try to install the latest version
of virtualenv for Windows, I get the message that Python 2.5 is not
found.

How do I install Python 2.5 (or anything else except 2.6) so that I
use the correct version of Python at the correct time.

For example, do I need to edit the "path" variable each time that I
want to change versions from 2.5 to 2.6?