There's no need to pay for Avast. The free version is all you need. It's just as good as the paid-for version because it's exactly the same anti-virus program. (It simply doesn't have all of the bells and whistles that the full security suite does).
All you need alongside an anti-virus program is a decent firewall and an anti-malware program. Most people are happy enough to use the Firewall which comes built into Windows anyway but some people prefer to use a free third-party product, such as Comodo or Zone Alarm. The majority of computer owners also make use of a free anti-malware program, such as the free version of Malwarebytes Anti-Malware. (I use the Windows firewall, together with the free versions of Avast and Malwarebytes). There really is no need to pay for security software at all; most people don't.
However if you've already paid for one of the 'bells and whistles' versions of Avast, you can transfer your subscription by following the instructions here:
https://support.avast.com/en-gb/article/Transfer-Antivirus-subscription