What is a good antivirus for windows?

Something is wrong with my computer. I’ve done everything I can to keep it up and going and it just keeps acting up. All I have on it that could possibly take up space is music and pictures but I just took off all my pics saved them to a disc and I have to keep my music in itunes. So could someone give me some advice???? Also I’ve had someone using my computer thats a freakin idiot and I think they downloaded some crap on here I’ve deleted everything I didn’t recognize and it’s still a little nuts. I know this isn’t a tech site but I value everyones opinion on here and would rather ask fluther.