Which python version do I use with "virtualenv"?

[OldGrantonian]

I have Windows Vista Home Premium.

As a non-techy, I want to use "virtualenv"

I had Python 2.6 on my laptop.

I needed "easyinstall" to install virtualenv. During installation of
easyinstall, I got the message "Python 2.5 not found"

So I installed Python 2.5, then installed virtualenv

So I now have c:\Python 2.5 and c:\Python 2.6

On the virtualenv web site, the instructions for use are:

$ python virtualenv.py ENV

My question is, which "python" should I use on this command line: 2.5
or 2.6?

BTW: I am currently running a Python app using 2.6. I want to install
a beta upagrade of this app, without interfering with my current
setup, so the app developer recommends using virtualenv.