If the previous suggestions don't solve the issue then I would suggest you do a 'Clean Install' of Avast5.
As a starting point, lets ensure that all remnants of Avast 4.8 are gone.
For a clean Uninstall/Reinstall please follow these instructions (I have assumed you are using the free version):
1>Download the latest version of Avast, 5.0.677 Free AV:
http://files.avast.com/iavs5x/setup_av_free_eng.exe (English only) and save it to your HDD, somewhere you can find it again (if you didn't save your last download). Use that when you reinstall.
2> Download the Avast! Uninstall Utility, aswClear5 at:
http://public.avast.com/~forejt/aswClear.exe and save it to your HDD (it has uninstall tools for both 4.8 and 5.0).
• 1. Now uninstall Avast! (using add remove programs, if you can't do that start from the next step), reboot. When your system is up and running Reboot into ‘Safe Mode’
• 2. run the Avast! Uninstall Utility from
safe mode separately, first for 4.8 if previously installed, then reboot into
‘Safe Mode’, and then for 5.0. Once you have completed, reboot into ‘Normal Mode’.
• 3. Install the latest version and reboot.