I NEVER use the internet on this computer.
how do you update avast?

Through the thing that pops up in the corner. (What I mean by not using the internet is not opening any browser. I used the internet once when I first got the computer, but that was to go straight from Bing to the Avast website. Other than that time, I do not use the internet.) My brother suggested avast gave me the virus, but I highly doubt it as a. I have had it for 2+ years and b. I only
got a virus result from a scan after the SD card. I have also not been able to find any instance online of someone getting a virus from avast. Should I go online and download malwarebytes as well?

I am sorry about misplacing the post yesterday. Also, thank you for the help. It is nice not to be called a liar (because it totally would be beneficial to lie to someone who is fixing your computer.)