A clean install could help as there have been several problem's on some system's when installed over the top and updated though GUI.
Sounds right to me
Please follow these instructions if you do not recall.
Download the latest version of Avast (5.1.864) for the product you need and save it to your HDD:
Free AV:
http://files.avast.com/iavs5x/setup_av_free.exePro AV:
http://files.avast.com/iavs5x/setup_av_pro.exeInternet Security:
http://files.avast.com/iavs5x/setup_ais.exe Then download the Avast Clean up Utility
http://www.avast.com/uninstall-utility and save it to your hard drive where you can access it easily.
Please go to Start/Control Panel/Add and Remove Programs and choose Avast and click Remove and then Uninstall. Once the uninstall process is completed, Reboot your computer and when up and running, Reboot again into 'Safe Mode' and find the (aswClear v5) download you saved to your hard drive and run the Clean up tool. Make sure you first choose the version you want to cleanup and then run the tool, your computer will automatically reboot. Once up and running in Normal Mode, Install the latest version of Avast you saved to your hard drive and all should be fine.
Please remember you need to choose the version in the Clean up tool that you have had installed on your system. If you have had both v.4 and v.5 installed, then you need to follow the ‘safe mode’ process twice; once for each version separately.
If you have any problems please feel free to let us know.