Possible malware slowing my PC?


Mar 9, 2015
I built my PC a year ago with no problems. My free Antivirus virus, avast wore out a day before I had my problem. Anyways I was using my computer, attempting to load CSGO and all of sudden everything went to a crawl, when I pulled up control, alt, del it said my "disk was at 99% usage" then a pop-up window said windoes had stopped working, so I shut my computer down. When it finally loaded back up it said it was Defraging my computer. I can log into Windows but every thing loads for about 5 minutes before it does anything. The computer is so slow it's un usable, I finally got it to reactivate my Antivirus but I can get past that. What should I do?