The problem with free versions of anti-malware software is that while they usually have the same search engine like their full commercial counterparts they usually don't have a web-scanner. That means that they will only get to action when a virus has already infiltrated your system and maybe corrupted it, but they won't prevent your system from getting infiltrated. Once a virus has infiltrated your system it's mostly very hard to get rid of it. Anti-malware software often just deletes the corrupted files and you still can't be sure that the virus is completely removed. So you would have to reinstall your whole system to be sure. Web-scanner scan the Internet traffic before it gets to your browser and blocks viruses. So those ~ 15 bucks for a full commercial version are worth it.
I use two approaches to protect my system:
One is the anti-malware software. I use kaspersky internet security 2012 which had the overall best ratings in different tests.
And the other is to run my browser (Opera, which is said to have the least security holes) only sandboxed with Sandboxie