Sorry to hear about your issue. That said it is a known issue with many complaints posted here on the forums.

If you need to make a “clean” install here’s how…

  1. download and save http://www.avast.com/uninstall-utility
  2. download and save http://files.avast.com/iavs5x/avast_free_antivirus_setup.exe
  3. run the uninstall utility from step #1 You will be prompted to run in “safe” mode, do so.
  4. install avast from step #2
  5. reboot and register 8)