Just to rule out somethings…did you use a cleaner like CCleaner?
http://filehippo.com/download_ccleaner/
Did you follow the instructions http://www.avast.com/uninstall-utility
You may have a corrupt download. If all else fails try downloading a fresh copy http://forum.avast.com/index.php?topic=88272.msg706930#msg706930
What OS are you using? What other security software do you now have or ever had? What do you use as a firewall, now or before?
Other security software may cause conflicts with avast.
To give further assistance we need more basic info. 