^^^ The same thought occurred to me.
Click the little Avast icon in the bottom right-hand corner of your screen. (You might have to click on 'Show hidden icons' first). If you're using the free version of Avast you should see a window like this one
http://www.upl.co/uploads/Avast1581449844.jpg
with the word 'Free' up at the top left and 'Upgrade' down at the the bottom left. (You can ignore the warning, with a 'Resolve' button, across the button; it's only trying to sell you something).
If you don't see 'Free' and 'Upgrade', you're using a free trial of the full commercial version of the software (rather than the totally free one). As Paul suggests, you can then uninstall the full version and install the free one. [NB: I recommend restarting your PC in between uninstalling and reinstalling].