Computer virus warning

If someone claiming to work for a reputable company calls or e-mails with a warning that your computer has a virus, you might be worried enough to follow the person's instructions: Go to a website, download a program, or grant remote access to your computer.

Don't do it. The warning is a scam, and the scammers are after password-sensitive information or credit-card data that will let them rack up unauthorized charges.

The scam has become so widespread that Microsoft has studied the problem in four countries, including the U.S. The study found that scammers stole an average of $875 from victims and caused $1,730 in damage to their computers. In October, the Better Business Bureau issued its second alert about the scam.

What to do. Ignore messages or ads saying that your computer may be infected, and never click on a link that leads to a website (or downloads a program) promising a free computer scan. Instead, install a virus-protection program such as the free AVG Anti-Virus 2012, and make sure it's set to update automatically to protect against the latest threats.

Before assuming that a computer's slow operation, frequent crashes, or error messages are due to a virus or other malware, search the Web with a description of the problem and look for responses from other users on message forums, or contact tech support.