Avast doesn't change the system interface so i would be inclined to think you have some other issue as your the only person that has reported this happening, what av were you using before avast and how did you remove it ? have you used and registry cleaning tools that may have removed your setting's ?
I was using the previous version of Avast. I didn't remove it. When the pop-up advising me to install the new version interrupted me for the umpteenth time, I shut down all open apps and let it install.
When I was trying to restore a desktop icon that had reverted, Avast prevented me from opening a program that makes changes to icons, until I "allowed" it. The program is FolderMagic and I've used it for years. So far, that's the only app I've tried to open that it has interfered with, but everything takes a lot longer to load. I was happier with the earlier version. Is there a way to go back to it?
No. I do not use registry cleaners. Horrible things.
I haven't made any other changes (counting Avast Update as a change) in months. The only program I've installed since January, was PDF to Word.
In answer to the other two comments:
There is nothing new in the virus chest. Received no notification of anything fishy.