I'm an OAP too and I know the financial strain that entails believe me !
I saw this on on Amazon UK: McAfee AntiVirus Plus 2015 - 1 PC (PC): Amazon.co.uk: Software very cheap.
In that case I would uninstall the trial before installing it.
The procedure is outlined here: How to uninstall or re-install supported McAfee products using the Consumer Products Removal tool
Just don't plug in any new USB sticks, or surf the Internet, and it can take as long as it likes - just being connected to the Internet (through a router) doesn't expose you to much risk - it's surfing and downloading which is risky.