Please follow this article and let us know what happened: https://support.avast.com/index.php?_m=knowledgebase&_a=viewarticle&kbarticleid=580&nav=0,2 [Windows 7]

Edit: For Windows vista : https://support.avast.com/index.php?_m=knowledgebase&_a=viewarticle&kbarticleid=581&nav=0,2