There's a website that's designed to outfit non-governmental organizations in politically dodgy countries (like Zimbabwe) with enough free or low-cost computer security software to keep them in good shape. -http://security.ngoinabox.org/ (putting in the hyphen at the beginning of this URL prevents it from hyperlinking, so you will need to copy it, without the hyphen, into a browser line)

They happen to recommend avast! as an antivirus, maybe just because it has a 30-day free version. Anyways, you can read what they say about it and other programs: -http://security.ngoinabox.org/en/avast_main

I have one of the big commercial programs that updates all the time, but I still got a trojan recently because I was using Internet Explorer (IE) for a quick lookup. Most trojan writers use IE vulnerabilities to get their malware into your computer. Firefox is much safer to use as a browser in general. To ditch the trojan, I had to download the free version of Malwarebytes, -http://www.malwarebytes.org/ The thing is, trojan writers now also aim to disable or take over the big antivirus programs like McAfee and Norton when the malware starts to run, so your antivirus may become part of the problem rather than part of the solution. Malwarebytes saved my skin when my antivirus was floundering helplessly.

That first link I gave also has some other security software that is free and very handy to use. There, I am talking about 'real' security, like wiping of your browser footprints and encrypted file storage.