I doubt that it is a virus attack, probably just some corruption in the avast files.
However, there has recently been an issue about permissions, but I don't think that it effected the version you are using in this way. Previously it was found that some of the avast folders had to low a level of permissions so it was theoretically possible for anyone to change files in the folders. A resolution to this was incorporated into a beta release and then into the latest release version 4.7.817 and I believe there were a number of people who experienced something like you are querying.
So a forum search or browse the avast 4 Home/Pro topics list may fond more on this issue.
I would suggest that you now download the avast! Uninstall Utility,
find it here. It would probably be best to also download the latest version of avast and save it to your HDD, somewhere you can find it again.
Try the windows add remove again and uninstall avast (even if it fails), run the avast uninstall utility, reboot, install the latest avast version, reboot.