Maybe install as required and then maually create the folder C:\python24 and cut/copy the files from the actual install directory to your newly created one?
I believe that Python is automatically searched for by location rather than by the values in the windows environment variables. However, if my suggestion doesn’t work then you could always try combining my suggestion with editing the environment variables.
Edit: I believe that the relevant part of the environment variables for Python is called PYTHONPATH. Multiple versions of Python have always played well together on my systems though, which is why I think that Python is searched for by location rather than by the system variable. This might be a clunky way of switching between versions, but could work. I hope you find a way around your problem.
Edit2: I’m currently running Windows 7 and the window for editing the path variable can be found by bringing up the program menu (bottom left, I’m not sure what it’s actually called) and typing “path” (without comment marks) into the search box. After bringing up the window you might need to hit the button called “Environment variables”.