COMMENTARY--For the past few days, we've been fighting spyware here in the office. One of the computers somehow got a weather-tracking program on it that, even when uninstalled, somehow kept showing up. Worse, it was one of those ad-supported programs that watches what you do on the Internet in order to send you pop-up (or pop-under) ads.

I'm not quite sure how the weather app pulled off the resurrection feat. But I do know how I killed it and, in the process, discovered a lot more spyware on the machine. Today I'll tell you how I did this and what I learned.

For those of you who are still blissfully ignorant of this scourge, spyware (aka adware) is software that comes bundled with a free application you've downloaded. Some spyware programs watch what you do so they can present advertising based on your online behavior; others simply track your online behavior for whatever other nefarious or innocent purposes their creators have in mind.

There's nothing illegal about this, and some spyware-supported programs (such as Gator's electronic wallet) are actually quite popular. On the other hand, some people don't like the idea of software moving itself onto their computers, watching their online moves, and reporting those moves back to some advertiser.

If you have spyware on your machine, you're almost certainly getting more advertising than someone who doesn't. That alone is, for me, reason enough to remove it from my system.

The most notorious adware company is Gator Corp., which describes itself as "the leader in online behavioral marketing." Gator claims to make it possible for consumers to download "some of the Web's most popular software--for free."

"In return for receiving free software, consumers agree to receive targeted promotions/ads from Gator advertisers through the Gator Advertising and Information Network (GAIN)," the company Web site reads. "GAIN occasionally displays various forms of pop-up ads in a separate window on users' computer screens. These GAIN ads are displayed based on the interests of the computer user as reflected by their Web surfing behavior and are not sponsored or endorsed by the web pages being viewed."

My reading of that: Gator basically hijacks Web audiences and presents competitive advertising to them. Further, I know lots of people who ended up with Gator software on their machines without realising they'd agreed to download it. Gator has been very good at sneaking onto people's machines.

Gator isn't alone. There's lots of spyware out there. If you regularly install downloaded software on your system, it's worth checking SpyChecker, a database of apps that carry adware, before you download.

Fortunately, there are also several apps that'll help you find out which programs are on your system and help you get rid of them.

If you think you may have spyware on your system, I recommend a program called Ad-aware to find and remove it. That's what I used to get rid of the spyware on my co-worker's machine. Developed by Lavasoft (a Swedish company that for some reason uses a German domain name), Ad-aware is available free for personal use.

Ad-aware scans your system for known adware/spyware and then gives you the opportunity to quarantine what it finds. If for some reason you remove a file you actually need, it's easy to bring it out of quarantine.

I've found Ad-aware to be quite effective, though I usually follow up by running Norton SystemWorks to clean up any mess left behind in my system registry.

In a recent column, my colleague Robert Vamosi mentioned a couple of other programs that I've tried. Spybot may strike some peoples' fancy, but it didn't strike mine. It presents you with a list of potentially troublesome browser cookies and offers to quarantine them. Webroot's Spy Sweeper is easier to use than SpyBot, which isn't surprising since it's sold as a subscription service.

Both of these programs found a lot of spyware on my system. Spy Sweeper identified three programs and more than 35 suspect cookies on my machine. The programs I recognised: HP uses BackWeb to send me updates, Microsoft uses Alexa to offer me Web pages related to the one I am looking at, while Wild Tangent provides music visualisations. I quarantined all but BackWeb.

As for the cookies, I killed all of them. Spy Sweeper didn't find any from places I recognised, so I took the program's advice and quarantined them all.

You can also protect yourself by using any of the several commercially available Internet security software packages. ZoneAlarm is available in both commercial and freeware versions (look way at the bottom of the company's download page for the free software) and Norton Internet Security is also quite popular. Both will prevent spyware from reporting back to wherever it came from.

After using Ad-aware to remove what actually turned out to be several different kinds of spyware from the machine, I installed the Norton software and things have been just fine since. It also helped to advise my coworker to stop downloading free software without checking SpyChecker first.

What do you think? Are you plagued by spyware? What do you do about it? TalkBack below or e-mail edit@zdnet.com.au.  

Copyright © 2009 CBS Interactive, a CBS Company. All Rights Reserved.
ZDNET is a registered service mark of CBS Interactive. ZDNET Logo is a service mark of CBS Interactive.