This is not the first time that a user posts about this issue, but the definitive cause was not found in that topic.
My suggestion would be to DIRECTLY use the removal tool for Avast located at
http://www.avast.com/uninstall-utility.
I suggest booting into
Windows Safe Mode directly and running the removal tool, selecting the correct path where Avast is installed and the correct installed version.
If you used to had a previous version of Avast before (for example if you updated from Avast 5.x to 6.0), then reboot AGAIN into Safe Mode (and please, don't skip this reboot) and run again the removal tool for the previous version too.
Repeat the reboot for each version of Avast you ever had, and run again the removal tool, selecting the correct version each time.
Please skip the "Add/Remove Programs", and avoid running the removal tool from Windows Normal Mode so to get the best results.
Once all versions of Avast are removed, search the TEMP path and delete any *avast* folder, and reboot again.
Now install the latest stable version of Avast (with right click -> "Run as Administrator" in Vista/Seven), and
reboot immediately (before updating, before configuring Avast and before registering it).
Test it during several days and report back.
Note that "some" amount of those temporal files during some time is normal, until Avast performs a "clean up" by itself, once in a while.
If you used to HAD other security tools before, they should also be removed. Some specific removal utilities (but not all) are listed at
http://thewebatom.net/uninstallers/security-software/.