Best free antivirus I ever used was AOL's free antivirus a bunch of years back. Back then, it was a slightly stripped down version of Kaspersky. While it is possible to pirate Kaspersky, for those people who don't like to pirate things, this was a great deal.

Then AOL switched it to McAfee and I dropped it for Avast. No particular reason for picking Avast; at the time, if you didn't want to pirate nod32 or kaspersky, avg or avast were your best choices for free antivirus, and I liked the name more for avast. I've heard good things about MS's antivirus package since then, but I feel comfortable enough with a combination of Avast/noscript/adblock and safe browsing habits that I haven't felt a need to switch.

Side note: I've gotten maybe ten pieces of spyware in the two years I've been using adblock, compared to 50+ per month beforehand. If you have a spyware problem, I heartily recommend it.

>What's the best anti-virus software you've ever used?
NOD32.
>Did you pay for it? Was it free? Was it a free "trial version" that expired after a certain amount of time?
Nope, nope, and it has one but nope.
>How often did you have to update it?
Errday.
>Have you ever used it to get rid of a virus, and if so, was it able to kill the virus when other anti-virus programs could not?
Yes and yes.
>Was it good at detecting viruses on its own? By this I'm asking if it alerted you when a virus was attempting to infect your machine, rather than you running the program manually and detecting a virus.
Yep. I think a scan only ever picked up something that the proactive scanner missed twice.
>How did it compare to other anti-virus programs you may have used?
It was the least system intensive, the most thorough, the most up to date, the last intrusive, the quickest updating, etc etc etc
>Do you still use this program?
When in Windows, yessir.
>Optional:
It's got a 30 day free trial that is literally the entire full version free for 30 days. After that, it cuts off, and you have to input a valid username and password to get it to turn back on.
Or you could just keep downloading free trials under different email addresses.