Minor nit in Py24 installer: Deselecting register extensions de-registers existing associations

["Martin v. Löwis"]

Mike C. Fletcher wrote:
> After installation, there are *no* associations with Python files, that 
> is "assoc .py" on the command line shows that there's no "Python" file 
> type remaining.

I'll see what I can do about this. This extension-registration turns out
to be really tricky, as MSI does not support conditional installation
of extensions out of the box.

Regards,
Martin