Which antivirus should I use?

As a resident tech person I get asked this question a lot. Here is a very short and simple answer.

Use Microsoft Security Essentials.

That's it. Microsoft has solved this problem. Download it, install it, and do whatever is says. Then uninstall any other antivirus you have running.

A slightly deeper dive: Norton and McAfee cause a lot of problems and you should uninstall them whenever you buy a new computer.

AVG would have been my previous recommendation. Its good, free, and efficient. But its not the best solution anymore. If you don't feel good about using a Microsoft product for some reason, this is still a great alternative.

These answers apply to a PC. I am not aware of a reason to run antivirus on a Mac or Linux computer right now.