Update 9/04/2014: Read my newest post about applesecurityalerts.com and Apple scams:
Mac Q & A: Is applesecurityalerts.com a Scam?
Treat anything that pops up in a browser like that as a scam. And the “Apple Security Alert” and “Apple Security Center” is definitely a scam. It’s a variation on lots of different Windows virus scanning popups, which are all scams, too.
What the website wants to download is malware to your iMac and get you to install it and then buy it by asking you to (or demanding you) enter your credit card number. But it’s a scam, the software is fake, and you will be ripped off.
The popups and software go by different names, like MacDefender Antivirus and Apple Security Center and MacGuard.
On OS X, you will always be asked for your admin password to install software, and that’s the give away that you’re installing something. If a popup asks for your admin password, it’s a bad idea to enter it.
The only legitimate time you will need your administrative password is for Apple’s own Software Update or for when you install software yourself from a CD or from a download you trust.
The problem with the latest MacGuard malware is that the installer can run by itself once downloaded. That’s because many users have Safari set to automatically expand file archives on download. It’s a good idea to not allow that. But you can change that. With Safari running, go to the Safari menu and select Preferences, and under the General Tab at the bottom uncheck “Open safe files after downloading.”
Malware for OS X will start becoming more common. And Apple released (on 5/31/11) a software update that will prevent the malware from installing and remove/uninstall it if it is installed. See About Security Update 2011-003. Run Software Update to get that update.
You should have software update set to automatically check and download updates. In the desktop, go to System Preferences under the Apple menu, and in that, go to Software Update icon in the System row. There you can tick “Check for Updates” (weekly is a good idea) and tick “Download updates automatically.” If you have never run software update, click “Check Now” and let it run.
If you did install MacDefender, Apple has a new KB article on how to remove it. See How to avoid or remove Mac Defender malware.
And if you want to run antivirus anyway (which will protect you and also help prevent passing on viruses to Windows users, too), you can try the free and reputable Sophos Antivirus for Mac. See https://www.sophos.com/en-us/products/free-tools/sophos-antivirus-for-mac-home-edition.aspx