Click here to protect your computer
The absolute worst thing you can do is click there. At best, these are shady advertising tactics that mislead consumers and scare them into buying software or services they may not even need. At worst, they're the product of malicious criminals who want to infect your computer with a Trojan horse and corrupt your data and/or spy on you.
These Trojan horses are often set up to log your keystrokes (keep track of everything you're typing) so that the criminals can steal your logins and passwords. With so much online banking and online bill paying, you can imagine what might happen if someone had access to all your passwords. Consider your bank account emptied, and credit cards maxed out.
So what do you do if one of these mysterious boxes pops up? Whatever you do, don't click on the box. Try to close it, and make sure that nothing is being installed on your computer. When in doubt, shut off your internet connection until you can close out the box and restart your computer if necessary. Install popup blocking software and a firewall (legitimate stuff) that will help protect your computer in the future.
Tracy L. Coenen, CPA, MBA, CFE performs fraud examinations and financial investigations for her company Sequence Inc. Forensic Accounting, and is the author of Essentials of Corporate Fraud.