This very helpful tip was offered up by The Remote Help Expert
Fake or rogue antivirus programs, also called scareware (you'll see why in a moment), are applications that pretend to detect and get rid of viruses, while actually being malware themselves, or being completely useless other than to scare the user into paying to download or unlock the full version of the "antivirus" to "remove" the "infection" the computer is plagued with.
Be very careful with this, as it is currently the most common type of infection going around - a rising trend. Assuming you have an antivirus solution in place - and I hope you do - know how it behaves and learn to recognize that it is not what is making an alarming pop-up window come into view all of a sudden and tell you there is an infection in your computer, and maybe prompts you to do a scan or simple starts a fake scan with lots of alarming results.
Once you've learned to recognize a fake antivirus in action, you have only two valid choices, and I'm deadly serious. The choices are based on your personal knowledge of malware and how to get rid of it. A) If you're not sure what to do, save any open files, turn off your computer and contact an expert. B) If you know what to do, well, do it!
I'll tell you what are NOT valid choices though: 1) Clicking on the "OK", "Scan", "Yes", "Download" button that the suspicious window contains, 2) Even clicking on the "X" on the upper right corner of the window to get rid of the pop-up, 3) Entering your personal and credit card information to purchase the "antivirus" so you can disinfect the computer, 4) Ignoring it hoping it will go away and continue using your computer. All those are invalid options.
Here's a list of trusted antivirus software vendors - you can use it as a starting point to recognize the legit ones: http://www.ccssforum.org/trusted-vendors.php
Hope this helps.