I noticed the topic about Avast and possible problems with "Unauthorized Changes To Windows". I just got that error message for the first time today. I am going to experiment with shutting down and disabling Windows Update, and see what happens. Windows Update often times downloads stuff to verify that Product Activation was not tampered with, and Avast 6 must clash with one of those verification tools that Windows Update downloads, so I am experimenting with disabling the Windows Update Service and seeing what happens. So I would recommend to other Avast users that are having this problem that, after re-installing Windows, shutting down and disabling Windows Updates, before installing Avast. That just might solve the problem.
With a good anti-virus program such as Avast, Windows Update would be quite redundant I would think, since Avast would protect you from malware and viruses.