LOL, fun isn't it? As a guy who uses the computers that others discard for these reasons, I have spent days scanning before I can turn it into a usable/safe computer.
Have you checked your task manager to see what is using up your resources? I once accidentally downloaded a program that installed with a ancillary program that ran continuously in the background to “monitor” my web usage. It was also using about 80 percent of my processing and slowed everything else I was doing to a crawl. I deleted the program and things got back to normal.