|Partial Comparison of Antiviruses, Click Picture for Larger View|
For the past 4 years, my answer has been the same. I always recommend Microsoft Security Essentials to PC users for several reasons. Microsoft Security Essentials is made by Microsoft, which means it's less likely to slow down a PC or have a negative interaction with other installed software. It also provides robust protection from viruses and malware, protection so strong, Microsoft was forced to provide the software separately from its Windows installation software to avoid a monopoly suit brought by antivirus manufacturers. ...and of course, Microsoft Security Essentials is free, and if it weren't a great product that wouldn't matter, but as it stands, free is just one more bonus.
There are other great antivirus programs out there and reasons to use them. Kaspersky Antivirus provides excellent e-mail protection. F-Protect products are updated meticulously. AVG by Grisoft is quick and provides a great service as well. Norton is one of the few antiviruses I recommend against, only because it tends to slow down machines significantly (in my experience) and I've had a lot of issues with its firewall malfunctioning.
In the end, the decision is a personal one. Some people have a favorite antivirus. For some, price is a deciding factor. Other users don't care what it costs if its easy to use and has great customer service. For a full and up to date comparison of all major antivirus softwares, try the Wikipedia article here: http://en.wikipedia.org/wiki/List_of_antivirus_software