Having an effective antivirus program on your PC is a bit like getting children vaccinated. It helps keep your computer running clean and uninfected, and it also helps everyone else, because your computer will never become a reservoir from which a virus spreads.
Current recommendation (as of 03-2015)
Based on recent (12-2014) results from AV-Comparatives.org , which is an independent lab that tests all available antivirus programs, my current recommendation for PCs is Webroot SecureAnywhere , and for servers, it is ESET File Security for Windows Servers.
Explanation & recent antivirus history
Prior to this change, I had been recommending Microsoft's Security Essentials for years. While Microsoft's antivirus solutions have never been as effective as their competitors, during the period from 2011 thru approximately June of 2014, they had been effective enough to do the job. Also in their favor, they are free, they don't slow down the systems where they are installed, and they run quietly, instead of constantly popping up with questions.
As recently as December of 2013, Microsoft's antivirus solutions were rated at 91.9% protective - and that mix of reasonable effectiveness, low impact footprint, and no cost made their solutions an easy choice.
During 2014, however, the performance of Microsoft's products declined significantly, until the December 2014 testing revealed a low, 83.5% protection rate.
At that point, I tested both Kaspersky and Bitdefender. Both have excellent reputations, and consistently score at the highest level on tests. But I found that Kaspersky, which highly effective, had too heavy a negative impact on the machines where it was installed, as well as frequently popping open windows asking if it could do this or that. Bitdefender was much better behaved, so it got the nod.
For those looking for a free antivirus solution, my current recommendation is AVG Antivirus Free . As of 12-2014, it was found to be 97.5% protective. I have found that it has a significantly more negative effect on system performance than Bitdefender. It will occasionally pop up nag screens asking that you purchase the full product, and it will also want you to install the latest version frequently, during which time it will also try to get you to purchase the paid version. But despite these annoyances, it is an effective program.