Good grief. Whatever you do DO NOT listen to these people. Get rid of Avast, yes, but DON'T just rely on Windows apps to safeguard your PC. I suggest you get ESET Smart Security, it's the best firewall and anti-virus out there and make sure you do scans every week at least.
Reading these posts above filled me with almost as much horror as reading on the Oculus Reddit not so long ago that someone is using a completely un-updated version of Windows 7. 😮
As someone that used to do system admin and IT support hearing or seeing these sorts of things gives me nightmares lol
My PC is like Fort Knox for good reason B)
I also make sure I completely format my OS hard drive and reinstall Windows at least once per year too.