Both!
There's a genuinely free version but you have to take care not to click on a link which takes you to a trial version of their full commercial security suite. Use the ORANGE button here:
http://free.avg.com/u...ee-antivirus-download
(and take care that you're not then tempted by a link to a trial version).
I used AVG Free for many years but, like several others on here, I found that the most recent versions caused a dramatic slowing of my computer. Like many others, I've now switched to the free version of Avast!
http://www.avast.com/...ee-antivirus-download
(Once again, take care to follow the correct links).
However, ChuckFickens (who works in the IT industry) is among a growing number of fans of Microsoft Security Essentials (which is also free)
http://windows.micros...s/security-essentials
Take your pick!
Chris