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