If you are using a computer with Windows installed, just use Microsoft Fax and Scan. Normally pre-installed. If using Win 10, just press the magnifying glass next to the start button and type fax in the search bar....
That sounds like a good start. Maybe make sure your Avast is up to date and that it runs fairly regular scans. Scan one more time with Avast after you are sure it's up to date. Reboot won't do any harm and then see if the pop ups go away
OK.. a bit of useful advice..
1. Download malwarebytes (the free/trial version is fine), install and scan your computer https://www.malwarebytes.com/mwb-download/
2. Make sure your current antivirus software is up to date. If you havent installed one, I am guessing you are using Windows, so...
Pick any from this song..
Vulcanize the whoopee stick
In the ham wallet
Cattle prod the oyster ditch
With the lap rocket
Batter-dip the cranny axe
In the gut locker
Retro-fit the pudding hatch, ooh la la
With the boink swatter
If I get you in the loop
When I make a point to be straight...