Right after upgrading to avast 6, I’ve consistently been getting this popup (on Vista SP1): http://i52.tinypic.com/eafskk.jpg
I can also not access the control panel which closes immediately after opening.
I can confirm that the problem is being caused by avast as uninstalling avast solved the problem, and reinstalling avast brought the problem back. (I repeated this process twice to be sure).
All users also report the problem was solved when avast! was uninstalled. (all threads mentioning “Error: 0xc004D401” and “An unauthorised change was made to Windows” relate to the said issue).
Uninstalling the behaviour shield had no effect. (I did reboot)
I installed Vista SP2 yesterday and now things are working fine.
Still if any developers are here, you should look into the problem for all the SP1 users out there.