I've used AVG for years & I swear by it (after 30+ years in the business). It's as good if not better than the paid-for Mcaffee etc and performs better & faster. Plus you can schedule it to scan your hard drive daily whilst you are asleep and then automatically shut down the pc. :-).
p.s. I also use the free zone alarm and Spybot.
Sounds like you have a bigger problem on your hands. You may have to reformat your drive. There is a site you can check (I don't have the URL), but I believe it's called Malware Bytes and you can use what's on that site to fix your computer.