do you have any other antivirus progam installed…or had one
if so you should run the vendors removal tool to clear any leftover files that may conflict
tools found here. http://singularlabs.com/uninstallers/security-software/

avast removal tool here, follow instructions on website, run it for all versions you had and reboot
then try reinstall.

avast removal tool. http://www.avast.com/uninstall-utility
avast download. http://www.avast.com/download-software