I have purchased and successfully downloaded Avast Internet Security 2014 but cannot install, as I cannot open the attachment containing my Licence number/ID.
You do not need to open the license, you need to save it somewhere on your harddrive and when installing avast you should point to it.
you find how to do it videos here http://www.avast.com/en-eu/faq.php?q=video#searchForm
avast FAQ section http://www.avast.com/en-eu/faq.php
important, remove any previous used / factory installed AV according to vendors instructions before installing avast http://www.avast.com/en-za/faq.php?article=AVKB11#artTitle