The free versions are not as good as the paid for versions - otherwise why would they bother producing products aimed at the home market? Paid for versions usually have more comprehensive protection/scanning (viruses, malware, spyware, rootkits, spam filtering, firewall, etc.) in one product and you will get faster updates.
That said user education is the best defence and you can get pretty good protection by using a number of free products and keeping these up to date.
If you have multiple administrator accounts in use by many people who's internet usage you don't monitor or restrict, then you will need the best levels of protection and I would recommend a paid-for product.
If you use non-administrator account and you are the only one using the computer, you only frequent trusted/trustworthy websites, you have a firewall in place and you meet the licensing criteria then the basic level of protection provided by free products will suffice.
[Note: I am a provider of anti-virus products]