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Maybe this step-step for clean install may help: http://forum.avast.com/index.php?topic=143284.0
There are many threads with postings on this but I tried to create a complete How-To.
As DavidR said there are very few people whom required a O/S reinstall/repair and very many of those had other PC issues they are finding.
Also, it is not clear the ones that did were Avast caused…bottom line a clean install is always a good thing with complex software like an A/V.
It is also good to prep your machine too…see How-To thread.