There is nothing like a perfect Anti-virus, paid version or free version.
Every product is made with the aim to protect the users from the viruses, worms etc. Each product has varying protection schemes. It is difficult to say that one product is better than others.
What you have to look in a free product is that how frequently the anti-virus product company is updating the virus signatures through internet. What types of protections the product is providing?
Are they providing only anti-virus protection, or have an integral firewall also?
An antivirus product should be able to protect against viruses, worms which propagate either through shady websites, email, or casual swapping of files like games, software, music, movies etc.
No anti-virus can fully protect a computer user. Along with anti-virus products, the user should have safe web browsing habits. Any downloads from internet should be scanned before opening them. Any email from unknown persons with attachments should be straight away deleted without opening them.
Any external device like floppy drives (almost outdated), flash drives, external HDDs must be scanned first before accessing any of the files in it.
Have a good anti-virus product and regularly update the virus definitions on daily basis. All the important and critical files related to studies/work should be backup up regularly on an external HDD and kept in a safe place (Don't always keep the backup devices connected to the system).
These are some of the advices for a safe computing experience.
Now about the free antivirus products I have used, after trying out different products both paid and free, I have settled down on Comodo Internet Security Premium which is a totally free product available for both x86 and x64 platform of Windows.
This product has Anti-virus protection, Firewall protection, Proactive Defense and Security Protection and Sandbox feature.
I have no complaints so far about this product. The only difference between the paid version and free version is that paid version has customer support extra. Other than that both products are 100% same.
Anyway this is my personal opinion.