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Problem:
I'd like to run Python 32 bit (for compatibility with extensions) and 
Python 64 bit on Vista (for speed and 64 bit apps) on one machine. 
However Vista has an 'improved' installer for MSI apps, where if I 
install Python 64 bit first, I can't install Python 32 bit afterwards, 
because Vista's Window Installer says 'Another version of this product 
is already installed. Installation of this version cannot continue. To 
configure or remove the existing version of this product, use 
Add/Remove Programs on the Control Panel'
The converse of this is true, if you install Python 64 bit first, you 
cannot install Python 32 bit afterwards.
The MSI packaging needs to be updated.

It's not so clear that this is a bug. If you install both versions
simultaneously, they might stomp on each other's registry settings, at
least for the extensions. So this is rather a feature request.
Notice that the behaviour is not new to Vista. Ever since the early days
of MSI, two simultaneous installations of the same product were not
supported, and ever since Python started using MSI, it uses one product
ID per feature release.
In any case, this was fixed in r59112.