Statistics say that a Windows computer, without using an antivirus, is infected in about 15 minutes of surfing on the internet.
If you have a company, you already know that not every free antivirus is free for companies. Most antivirus softwares have a specific clause in license terms to only allow home users to use it. So for small businesses the options are limited. I would recommend Comodo Antivirus or Microsoft Security Essentials.
Home users have multiple choices when it comes to free antivirus software:
1. If you have a legal copy of Windows , on my opinion, by far the best as the performance / protection ratio is Microsoft Security Essentials (in my opinion, a Microsoft antivirus product for a Microsoft operating system is the best choice).
2. Another very good antivirus, with a good resource management and high level protection is AVG Anti-Virus Free.
3. Avira, Avast!, Panda, Immunet, BitDefender also have antivirus products that are free for personal use.