Which version of avast, free, Pro, AIS or Premier? What other security software do you have now or ever had? If uninstalled how was it uninstalled?
Security software includes anti-virus, firewall and anti-spyware/malware.
Two things you might want to try. Repair while in safe mode or a clean install.
For a clean install…
- download and save http://www.avast.com/uninstall-utility
- download and save the version you desire
Download locations
http://files.avast.com/iavs5x/avast_free_antivirus_setup.exe
http://files.avast.com/iavs5x/avast_pro_antivirus_setup.exe
http://files.avast.com/iavs5x/avast_internet_security_setup.exe
http://files.avast.com/iavs5x/avast_premier_antivirus_setup.exe
- run uninstaller from step #1 (you will be prompted to reboot into safe mode, click “yes”)
- reboot into normal mode
- install avast from step #2
- reboot and register

Watch out for the Chrome opt-out screen.