These days, it almost seems there are more bad scanners than good. Rogue scanners display false virus alerts, with instructions to buy the "full version" in order to remove the fictional infections. Wasting your hard-earned dollars on a bogus product isn't the only thing you need to worry about. There have been multiple reports from folks who fell for the scam only to end up with credit card charges much higher than expected - and often a series of fraudulent charges continue to appear on their statements. Victims typically encounter rogue scanners in one of three ways:
You can help mitigate your risk of exposure by reading email in plain text only and avoid clicking links or opening attachments in email received unexpectedly - even if it comes from someone you know. Stick with the good guys: for a free online scan, use one of these top online scanners. Keep your system patched to prevent exploit: use the free Secunia Software Inspector at least monthly to check your system for vulnerabilities that need patching.
Here's a list of 69 scanners you should avoid. Also see: Six Steps to Tell if a Virus Alert is Legit.