How many times have you heard "you get what you pay for"? Fact is "free" programs aren't as good or effective as ones you pay for. Consider that literally hundreds of new threats are introduced on the net every week not to mention the spyware that vendors use. To effectively put up a fight against this you have to have a way of knowing when these things hit (and time is critical so the sooner the better) and a team of programmers constantly on hand to figure out how these new threats work, figure out a "cure", then write a program to fix it. Do you really believe this can be done for "free"?
Now admittedly I don't know much about "free" internet security programs but here's what I've found during my brief research. Most of them don't "prevent" you from getting the infections. They only offer a free scan to try and clean up "after the fact" (which all of the paid ones offer for free anyway by just visiting their sites). I know from experience it's better to not get them in the first place than to get them then try to get rid of them. And the one's that do offer some sort of "protection" from getting them don't release updates on a timely basis leading me to believe they wait until the "paid" companies figure it out then they simply "copy" the fix.
The other thing about the freebies is I don't know of not a one of them that offer a complete internet security package (virus, firewall, spyware, anti-phising, etc) all in one. When you mix and match software to get the whole package you sometimes end up with them fighting each other and slowing you down just as bad as the viruses themselves.
Since internet security is so important to me I ALWAYS BUY a program that has a COMPLETE package and my experience says that Panda Internet Security is by far the best on the market. It also offers parental controls if you want to install it. It doesn't come piecemeal. When you buy it you get it all. It updates automatically every day and has a technology called trueprevent that detects threats not just by definition but by the way they act so that if a threat hits you can be protected even before an update is posted.
Just my opinion based on over 20 years of experience.