This has already been asked and answered at least a hundred times on Tom's Hardware. Next time read the forums first.
But the best free one would be Avira or AVG or Avast, take your pick. I prefer Avira.
As for paid, they're not much better than the free ones (if at all), but Kaspersky and NOD32 are both pretty good. If you want an all around anti-bad-computer-stuff program you could go with Microsoft Security Essentials. It is ok. Otherwise make sure to get an anti-malware and a firewall along with your anti-virus. Recommend Malwarebytes Free and PC Tools Firewall Plus, which is also free.
I think that at the moment Kaspersky is the best antivirus available. It is a bit more expensive than the others but in my opinion its well worth it. They have the highest detection rate of all the top antivirus programs and has been recommended by a lot of PC Techs that i've talked to. BitDefender is also good and a lot cheaper so may be a better option for you. BitDefender is also a lot lighter than Kasperksy and so uses less resources. If you have a lower end machine go for Vipre as it has the lowest resource usage of any of the popular antivirus programs and is still a great product. In my opinion avoid Mcafee (Huge resource hog) and Norton.
If your looking for free antivirus then AVG is by far the best!
The website is at http://free.avg.com
Theres some great antivirus comparison and review websites on the net. Take a look at these three:
dont know where you people get your information. are you paid for by these companies?
these are not known but Icarus and Trustport (4 scanning engines in one tyvm) are the ones with best detection rates.
Obviously theyre not free.
Also use sandboxie, to keep stuff in sandbox, so it doesnt spread in your operating system.