Q. My computer's been sluggish lately, and I seem to get a lot more pop-up ads. What's going on?

A. You likely have what's commonly known as spyware, though its makers prefer the term "adware." These are programs installed on computers, often without their owners' full knowledge, to monitor Web surfing activity and serve relevant ads.

One class of spyware, more properly termed "dialers," can even try to scam you by dialing "900" and international phone numbers instead of the local number given by your Internet service provider.

Another type, called "browser hijackers," changes the home page settings on your browsers, usually taking you to unfamiliar search engines.

Spyware can be tricky to find and remove, but fortunately, there are free software products available to get rid of such programs, which are written primarily to run on Windows computers.

First, you must think about how you might have gotten the programs in the first place.

Did you download and install file-sharing or other free software?

Then likely, you got spyware as part of the deal. It's a bargain you made when you installed the free software by clicking "yes" to accepting legal language you probably didn't bother to read. Buried in the legalese often is disclosure of such ad products.

But there are trickier ways spyware sneaks onto your computer.

If your Web browser's security settings are low, you might have gotten it simply by visiting a Web site. Perhaps you mistyped a domain name and got a search engine you never heard of, one that surreptitiously placed the spyware on your computer.

Pop-up messages that claim to help your computer run better are sometimes fronts for getting you to accept spyware installations.

A House subcommittee recently approved a bill that would require spyware makers to give consumers clear notice. Utah has passed an anti-spyware law, but it is being challenged in court by a company that considers advertising a free-speech right.

You won't want to wait for lawmakers or the courts to act.

Spyware can slow down your computer or Net surfing because it continually delivers pop-up ads. Some are not written well, so they can conflict with existing programs, cause computers to crash or otherwise use computing resources inefficiently.

In some cases, you can manually remove spyware by using the "Add/Remove Programs" option in the Windows control panel. But the less ethical spyware makers don't make removal that easy.

Instead, it's best to run a number of free software designed to find and root out spyware.

Start with Spybot Search and Destroy, written by a German programmer with help from friends and volunteers. Download it at www.security.kolla.de/, which also shows up at www.safer-networking.org/. It's free, though the author requests a donation.

Begin by making sure you have the latest lists of known spyware. Just click on "Search for updates."

Conduct a scan and remove any programs or files you don't need, keeping in mind that in some cases, spyware removal can also disable file-sharing and other software (I usually remove everything, and reinstall the free software again if it stops working). Then run the scan again.

No anti-spyware program is going to catch everything, so run another. Try the free version of Ad-aware from Lavasoft (Go to www.lavasoftusa.com). Again, make sure you have updated lists, by using the "WebUpdate" option at the top.

If your computer is so bogged down with spyware, you may have trouble downloading the anti-spyware products before your computer crashes. In that case, you may have to obtain the programs from another computer with a CD burner, then load the software onto your troubled computer using a CD.

Once you've removed all the errant programs, do a scan every so often in case new spyware has sneaked in since then. Just make sure you update the lists first, as spyware makers are constantly finding newer ways to get to you.