If you use the Online Reg, then there is no key…just Your Name and a valid Email address

On-line registration is very simple and it’s the preferred method for all users of avast Free Antivirus 5.x installed on a computer with standard Internet connectivity. To perform on-line registration follow these steps:
http://support.avast.com/index.php?_m=knowledgebase&_a=viewarticle&kbarticleid=459