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All the mice use the same SetPoint software IIRC so if it's not for it just download it from a different section.
You sure it was 5.8, and not 5.2 ?
I ask because the only hit I got via google that had any relevance was this thread.
Here is the link to 5.2, for the G500, G9, and G9x:
http://www.logitech.com/index.cfm/428/5750&hub=1&cl=us,en?section=downloads&bit=&osid=14