Do yourself a favor. Go up that in the top left corner of your screen. Click Software Update. Install Security Update 2011-003 (Free). You’ll thank us. All done? Alright, let’s talk about it. This Security Update has one purpose: to get rid of the Mac Defender malware.
You’ve probably heard that there’s something going around, stealing credit card numbers and the like. If you happened to end up with it on your Mac, it’s easy enough to get rid of on your own. Once you’ve installed the Update though—which you did already, right?—OS X will automatically search for and remove any known variants and let you know afterwards. It also checks daily for updates to make sure that no new versions can sneak onto your system. This is definitely one of those times when you don’t want to hit “Not Now.”