Is your computer stuck in first gear? Does it cough and sputter and crash more often than usual? And, by chance, are you seeing an unusually high number of pop-up ads?

If so, it's possible your machine harbors one or more programs known collectively as spyware, nasty little intruders that combine the worst features of e-mail viruses and spam into one package.

Like spam, spyware co-opts bandwidth in an attempt to sell you something. And just like viruses, spyware can slow down your computer, alter its settings without your permission, monitor your every move and transmit personal information about you to snoops.

Spyware isn't new to the Internet, but it is becoming more prevalent. According to a December 2002 report from research firm GartnerG2, more than 20 million people now have spyware, also known as adware, lurking on their machines, and those numbers are rising rapidly.

Some security experts warn that spyware, not viruses, is the biggest threat facing computer users. Whereas most businesses have set up filters to fend off known viruses contained within attachments -- and some users are learning not to click on attachments at all in order to avoid getting hit -- few casual users even know about spyware.

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