Is th AV you used before avast removed…using vendors removal tool to clear all files? http://www.avast.com/en-us/faq.php?article=AVKB11#artTitle
Remove your avast according to instructions here http://www.avast.com/en-us/uninstall-utility
Download and install your version http://www.avast.com/en-us/download-software
Insert license http://www.avast.com/en-us/faq.php?article=AVKB8#artTitle
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