Also called rogue security software, scareware is malware that has infiltrated your computer and uses pop-ups and other tricks to convince you that your computer is infected. The pop-up message encourages you to buy their software to eliminate the infection. Once you enter your purchasing information the people that authored the software use your credit card information for their own purposes.
It is becoming increasingly challenging to tell the difference between legitimate anti-malware products and scareware programs. The best way for you to protect yourself is to install an anti-malware program from a trusted vendor and keep it up-to-date. If you see a pop-up like the one displayed here, close the window, and run a scan using your trusted solution instead.