install on Windows 8,1

Avast Free Antivirus / Premium Security
1

I have Avast Pro installed on my old computer and can add up to 2 other computers. How do I install on my new Windows 8.1

2

first make sure any previous used or factory installed AV is uninstalled according to vendors instructions before installing avast
General: Uninstalling a third-party antivirus software http://www.avast.com/faq.php?article=AVKB11#artTitle

then download install and insert the license you have

avast FAQ http://www.avast.com/faq.php
avast FAQ videos http://www.avast.com/faq.php?q=video#searchForm