Have you ever seen a "free" offer to scan your computer for security vulnerabilities? The most common one that I get is a pop-up ad that reads "Your computer may be infected with harmful spyware programs. Immediate removal may be required. To scan, click 'Yes' below." It looks like a great idea. You're offering to test my machine for free so I know what, if anything, needs fixing. Doctors, mechanics, even the lawn guy offers that kind of free screening as a legitimate way to build a relationship with new customers.
Unfortunately, most if not all of these computer scanning offers are scams. They are rogue programs that will always report something that needs to be fixed or cleaned whether the flaw is real or not. They are designed to scare you into believing that there is something terribly wrong with your computer that only their software can fix.
Examples that attack Windows computers include SpySheriff, WinFixer, IEDefender and Cleanator. Interestingly, Mac users ran into this problem for the first time in January with a product called MacSweeper. MacSweeper is so "thorough" that it even finds flaws when it's run against a PC – flaws that can only exist on a Mac.
Most of these are simple attempts to con you out of money or credit card numbers. Some are more malicious and will load spyware onto the computer or even disable your existing antivirus programs.
Never run software from unknown sources. If you do suspect that your computer may be vulnerable, use your own anti-virus and anti-spyware software. Don't trust that "free" offer.
Note: The word "scareware" also includes more harmless pranks such as the program that pops up and says "Erase everything on hard drive?" with two buttons labeled "OK" and "OK". (Nothing is actually deleted in this prank.) Just ignore those pranks.