Is Your Computer Infected?

En español | At any given time, roughly half of all computers are infected with "malware" — programs that can steal files and passwords, hold your machine hostage for purchase of bogus security software, or enlist it into a "botnet," a network that makes it secretly send out spam.

See also: How to set up a wireless network in your home.

Is yours among them? You can't always depend on your antivirus software to tell you.

Each minute, some four dozen new strains of malware are created, many of them designed to elude antivirus programs.

And each day, about 8,600 new malware-containing websites are launched.

It typically takes two days for security software to be updated to block a new malicious website. So watch for these symptoms that may signal your computer's been infected.

Peter Dazeley/Getty Images

Do you know if your computer is infected?

• Slowness. Your computer suddenly takes much longer than before to run programs, access files or the Internet, or even shut down.
• Unexpected beeps or sounds. Or your computer's fan may suddenly kick into overdrive.
• Frequent pop-up messages. Be especially vigilant if these are warnings that you need to purchase software to remedy a security threat. (The software is generally fake.)
• Unwelcome images. Sometimes they're pornographic, sometimes they replace benign images such as photos on news sites.
• Disappearing files, folders or icons.
• Lost protections. Your antivirus software vanishes, your firewall becomes disabled, or you cannot download operating system or antivirus software updates.
• Mysterious messages. Your friends and family receive email messages from your account, but you didn't send them.
• Automatic start-ups. Familiar programs on your computer (or ones you didn't know were there) start unexpectedly, or you're randomly connected to unknown websites.
• Freezes. Your computer frequently seizes up or you can't leave websites or close your Web browser.

How to Protect Your Computer

If you think you've got malware, follow these steps:

1. Disconnect your computer from the Internet and its power source, then reconnect the wires and restart in safe mode. On a Windows PC, hold down F8 as soon as the reboot begins, then choose "safe mode" from the menu that appears. On a Mac, hold down the shift key while restarting.

Check for updates to your antivirus software, then perform a complete scan.

2. If your antivirus software is acting strangely or is missing, start your computer with a rescue CD — a special disc that contains your computer's operating system and allows you to start your machine even if your hard disk is corrupted.

Sometimes a rescue disk is supplied with your computer. Or use someone else's computer to create one from free downloads at any of these sites: AVG, Avira, BitDefender or F-Secure. (You should do this before you have problems.)

3. Check the website of your antivirus software vendor for additional information or software patches that will remove a particular malicious program.

If no malware is found, but problems continue, it's best to have your computer examined by a technician.

You may also like: How to maintain your computer.

Sid Kirchheimer is the author of Scam-Proof Your Life, published by AARP Books/Sterling.