Not sure if it will make any difference from uninstall/reinstall, but you may try avast! repair and reboot
For a repair of avast. Windows, Add Remove programs, select 'avast! Anti-Virus,' click the Change/Remove button from the pop-up window, scroll down to Repair, click next and follow.
OBS: i guess you have already removed any previous installed AV and used a removal tool to clean out any leftovers
Removal tools can be found here
http://uninstallers.blogspot.com/