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All versions are binary downloads for windows, from 
python.org

I have a 2.4, which seems not to be relevant.

I don't remember whether I had installed 2.5a1, though I 
think so.  If I had, it was successfully overwritten by 
2.5a2.

I had definately installed 2.5a2.

The problem came when I attempted to install 2.5b1 on top 
of 2.5a2.