I used AVG Free for years, but had some false positive issues with Spybot. Now I use Avast Free and it works great!
I'm using avg free version for 3 months and it works really great , except it detects cracks as virus lol..no other problem with it yet.
i use Kaspersky and it is a surprise but i am satisfied) remember that my boyfriend persuaded me for a month to use it)
Norton Antivirus and Internet Security is no doubt the industry leader in system protection from malware, Trojans etc. If you are willing to use Norton Internet Security for personal use, you can get 1 year free subscription license of NIS 2011 from this page. There is a nice giveaway going on this blog to let you get the box pack of this awesome software for free. I agree with other people as well that Kaspersky and other popular security software are also good for this purpose. One can also try Microsoft Security Essentials on Windows PC for complete protection and peace of mind.
Im using avast! but im gonna change it to kaspersky or avira since my friend advise me that these two are better.