Have you ever been casually surfing the Internet when, suddenly, a popup comes out of nowhere warning you that you have XX number of viruses? It would look like it came from Windows, and we wouldn't blame you for getting scared. This is actually a form of malware known as scareware - this software will not only infect your system, but it will try and trick you into using your credit card. The program does this by claiming that it will only remove these many viruses if you purchase the full version of its "antivirus software."
This sort of software can actually lock your computer down, blocking you from using it properly. To get around this, reboot your PC in safe mode and run a full scan of your PC using your normal antivirus software. It may take some time (and a lot of repetitions of this process), but eventually you will have a clean PC again.