I have some ideas, but I am not certain of a fix. This is what I would do, if you want to try.
The instructions assume that there is no malware on the computer capable of preventing a program installation. You would probably know if there was, because the computer would have other symptoms of problems.
Follow all the steps below.
-Download the latest Avast Home installer file from here: http://www.avast.com/eng/avast_4_home.html and save it to your desktop, or to a folder you have created.
-Download the latest Avast uninstall utility from here:http://www.avast.com/eng/avast-uninstall-utility.html and save it.
-Download the Avira Registry Cleaner from here: http://www.avira.com/en/support/support_downloads.html it is the number 6 download, and save it.
-Double check that you have no other antivirus installed. Look in “add/remove programs”.
-Disconnect from the internet.
-Disable Avast by right-clicking on the tray icon, and selecting “stop on-access protection”.
-Uninstall Avast, if possible, from the control panel> add/remove programs. Reboot. If this does not work, skip to the next step.
-Reboot into safe mode, and run the Avast uninstall utility. Reboot into normal mode.
-Run the Avira registry cleaner. Reboot.
-Locate the folder C:\Program files\Alwil software and delete it, if present.
-Install Avast antivirus by running the downloaded file. Reboot when prompted. I suggest you select “no” if asked for a “boot scan” during install; this can take a long time.
-Once set up is complete, reconnect to the net. Manually check for updates (if it doesn’t automatically do this).
-Within 60 days you have to register it. It’s free, but needs to be registered every 14months.
Hope that works. Should take about an hour, maybe a little longer.
[edited] missed a step.