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.

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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.

Do you know if your computer is infected? — Peter Dazeley/Getty Images

• 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.

Next: Do you think your computer has malware? >>