The question I have,
Why wouldn't the posters with the issue just install version 6? Maybe there is an issue with going from 4.8 to 5. Maybe the version of Avast 5 that you have or downloaded is corrupted. What I would do is the following:
1.) Uninstall your current version of Avast from Add/Remove Programs.
2.) Reboot your computer.
3.) Download the Avast uninstall utility to your Desktop or your Internet Downloads folder from here:
http://www.avast.com/uninstall-utility4.) Boot into Safe Mode as an Administrator and run the Avast Uninstall Utility.
5.) Reboot your computer.
6.) Get a fresh download of Avast 6 from
www.avast.com7.) Save the download to your Desktop or to your Internet Downloads folder.
8.) Install Avast 6 on your computer.
9.) Reboot your computer-wait about 2-3 minutes for everything to kick in with Avast 6.
10.) Open up Avast 6 with the desktop short-cut icon, Click on Maintenance, Register for your license Key, fill out the form and click Submit. That should take care of it.
Jack