The key to recognizing and avoiding these scams is to know what antivirus and security software is running on your PC and to know what its alerts look like. If you see any other pop-ups that appear to come from the Windows Security Center, don't click on them. Instead, click on the Start button on your PC, go to the Control Panel and then open up the Security Center.
The Windows Security Center will show the status of the security software on your machine. If you're running a third-party antivirus program, open that and check that it's running normally. If all is well, then it's time to get rid of the pop-up. Again, avoid clicking on the box itself and instead hold down the CTL-ALT-DEL buttons together and click on the Task Manager. Find the pop-up box in the list of running applications and stop it.
Find more information on scareware in a guest "Focus" article written by Dennis Fisher, editor of Threatpost.com