If you have "Open Safe files after downloading" enabled in Safari, downloading this file will auto-install this code.
In it's current incarnation, MACDefender shows up in the installed applications list, so can be uninstalled. If you have accidentally installed this, go ahead and uninstall it. I would not expect this "uninstall" option to be a good long term protection strategy. I'd suggest that OSX users disable "Open safe files after downloading", and also invest in a reasonable anti-malware suite. Installing a real anti-malware package is also a good idea (no matter what the Apple Fans say ..)
As in all platforms, downloading any code from the internet that suggests anything like "click here and we'll check you for malware" or "you have a virus, click here to clean your machine" or "click here to tune your computer to run faster" is probably a really bad idea. Almost without exeption, these sites are enticing you to help install malware on your computer, not protect it.
More detail here ==> http://www.macrumors.com/2011/05/02/new-macdefender-malware-threat-for-mac-os-x/