It’s easy to compare surfing the Web to swimming in a sewer. You never know what kind of little nasty is going to attach itself to you and make life miserable.

That’s what happened to numerous people who suddenly find $50 to $200 worth of calls to pornographic Web site numbers on their phone bills.

None had ever told their computers to dial the numbers. Unfortunately, these folks have a serious problem on their hands. They or their kids have inadvertently downloaded a program that disconnects them from their dial-up Internet service provider and silently dials a porn site.

Removing a porn dialer from your PC can be just as difficult as having the charges removed from your phone bill. Clicking on the program’s Uninstall icon will only trigger a program that reinstalls it. Even erasing the program files won’t work – they’ll show up the next time you start up your computer.

How does all this happen to people who wouldn’t be caught dead accessing a porn site?

Porn dialers became popular a couple of years ago when fraud and charge-backs were so common on adult Web sites that banks and credit card companies would no longer do business with them. So the porn promoters chose another venue – providing customers with software that would dial the operator’s ISP and charge the customer through his phone bill.

Unfortunately, the temptation was too great for the real sleazeballs who immediately figured out ways to sneak these programs onto user’s machines.

In some cases, the surfer downloads the program under the misconception that he’s installing a “free” porn viewer. In practice, it disconnects him from his regular ISP and silently dials a 900 domestic number or an overseas number that’s billed at $3 to $6 a minute.

But just as often, the download is the result of misdirection. Frequently it’s triggered by clicking a “Cancel” or “Close” button on a Window that pops up when you mistakenly type in a variant of a common Web address that’s been taken over by a porn site operator.

In the worst cases, the infection is triggered merely by visiting a Web site with malicious code embedded in the page.

There’s no easy method to rid your system of a porn dialer, but you can protect yourself. Install a good virus checker and a firewall such as Zone Alarm, or an all-in-one package such as the Norton Internet Security suite.

Another option is a spyware detector, such as AdAware, which comes to folks in both free and paid versions (www.lavasoftusa.com), and a new favorite, SpyBot Search and Destroy, a free utility that roots out adware, spyware, dialers and other pests. It’s available at spybot.safer-networking.de.