Scareware

A prime example of scareware is the pop-up you'll often get on your computer when web surfing that says your computer is infected with a virus.  You're prompted to click on the link to either clear the virus or purchases the necessary software to clear it.  Don't do it.  You'll either pay money for something that is either completely useless, or worse, you pay money and then download malicious software onto your computer.

Make sure you already have already have good antivirus software on your computer.  Be familiar with it, and what the interface looks like.  Make sure your antivirus software is set to automatically update.  Know your firewall settings, and download all security updates for your operating system.