Which anti-virus protection is best?

I am practically computer illiterate, so please excuse the ignorance. I have no idea how it happened, but it seems that both Norton & McAfee have both charged renewals to my credit card without my permission. The e-mails I got from both companies said something about me "agreeing to the automatic annual renewal", which is something I absolutely don't remember. I know that the person I hired to set up my computer advised me to get Norton, so that's how it initially got on my computer. But I never remember subscribing to McAfee at all?????? McAfee, however, seems to be the virus protection on almost every large website I order from. I have never seen Norton on a site from which I order. One other thing - could the fact that I seemingly have both anti-virus programs at work on my computer be slowing down its performance?