As you say you have had avast home for well over a year now, have you re-registered it as the VPS updates would be stopped after 14 months ?
Whilst that shouldn't stop the on-access protection from running, so Tech questions would be nearer to the true problem.
Have you changed anything recently as you say within the last two months, that would fall in line with the last program update 4.7.1001 at the end of April, 1st week of May 2007. So your reinstall may not have been wide of the mark, but you should try a clean reinstall if none of Tech's questions are applicable.
Download the avast! Uninstall Utility,
find it here and save it to your HDD so you can find it later.
Download the latest version of avast
http://www.avast.com/eng/programs.html and save it to your HDD, somewhere you can find it again. Use that when you reinstall.
Now uninstall, reboot, run the uninstall utility, reboot, install the version you downloaded, reboot.