welcome to the forum aka10
have you removed the previous antivirus program that was installed on your computer before you try to install avast?
which was that program Norton?
if so download the uninstall tool from nortons homepage and run it. sometimes it leaves files behind after you uninstall it.
https://www-secure.symantec.com/norton-support/jsp/help-solutions.jsp?lg=english&ct=united+states&docid=20080710133834EN&product=home&version=1&pvid=f-home
good luck.