Bill Davis, of Bismarck, knew something was wrong when he got his phone bill.

More than $300 in long distance charges.

To 900 numbers.

Almost all made during the day, when no one was home.

After a little research and numerous telephone calls, the culprit was found: A spyware redialer on Davis' home computer. A program unknowingly downloaded from the Internet that installed itself on Davis' unprotected computer system and then secretly accessed his modem to place calls to 900 numbers.

"Modem hijacking," as it's known.

And the tiny bit of software wasn't alone.

When Davis took his computer in to have the redialer program removed, the technicians at his Internet service provider said they found 6,500 viruses, more than 100 worms and 80 different adware and spyware programs.

A thriving community of automated digital thieves and con men, surreptitiously sending data, displaying ads, tracking Web access, monitoring keystrokes, recording computer activity and calling 900 numbers. All without Davis' knowledge.

Welcome to the long, dark alley of the Internet, a digital cesspool where you're a potential victim wearing a flashing neon "Steal From Me" sign above your head, much to the delight and interest of thieves and lowlifes lurking in the shadows.

You have something they desperately want: Money. Credit card numbers. Your identity.

And they will take it from you if you're not protected.

Accessing the Internet today without firewall protection and at least one antivirus program is like happily passing out your credit cards and personal information to any stranger you meet on the street.

Here are the four most common threats you face online and what you can do to stop them:

Spyware

"Spyware" is a program that performs various tasks on your computer, usually without your knowledge or consent. Some of these tasks may include collecting information on every Web site you visit, collecting all of your usernames and passwords, acting as a silent gatekeeper that allows other spyware to download and install on your computer.

Spyware also is associated with "adware," or programs that display unsolicited ads and pop up ads on your computer. In many cases, adware programs install themselves on your computer without your consent or in a way in which you give your consent without realizing what you are consenting to.

Many spyware programs are "bundled" with file-sharing programs or downloadable games. When you download and install a program, any attached spyware covertly installs itself at the same time. In other instances, spyware is quietly downloaded and installed when you click a link in a popup ad.

When you download a program, you agree to the terms in the End User License Agreement, or EULA. Often, buried deep in the lengthy fine print of the EULA is a statement indicating your acceptance of spyware or adware programs associated with the program you're downloading.

Look for a EULA or privacy statement before you download a program or before you install a program. Don't simply click the "I Have Read It" box or link without actually doing so. This is the legal "out" for many spyware companies. When you click the link, you are saying, "I've read and accept everything in the agreement."

If you cannot find a EULA or privacy statement, don't download the software or don't click the link.

Also, make sure a firewall is running on your computer. This is a program that monitors Internet traffic to and from your computer and stops certain traffic that may be virus related. Spyware and adware need to communicate with the Internet to send collected data from your computer or to pull in popup ads. Firewalls can identify and block unauthorized Internet requests from your computer.

Most current operating systems include a firewall feature - check the "control panel" in your system to turn it on and configure it to your needs.

If your operating system doesn't have a firewall, you can easily purchase one individually or as part of an antivirus software program.

BHOs

These are small programs used to customize and control various elements of your Web browser. Browser Helper Objects, or BHOs, include things such as search engine boxes on your browser toolbar, one-click weather displays, one-click image inserts such as emoticons that go into a chat window and so on.

Many BHOs are legitimate, useful services such as the Google Search Bar or The Weather Channel's "weather at a glance" box for your area.

But some exploit the control they have over a Web browser by gathering information on your browsing activities, copying your keystrokes as you enter usernames and passwords, pulling in popup ads or misdirecting you to Web sites you might not wish to visit.

In some cases, BHOs can corrupt important system functions on your computer.

If you use Windows XP, you can see what BHOs are installed on your browser by accessing the "Add-on Manager" in Internet Explorer. This will display what BHOs are attached to your browser and allow you to delete those you don't want.

If you're not using Windows XP, there are free tools such as BHODemon and HiJackFree that list installed BHOs and allow you to disable the one you don't want running on your system.

Viruses-worms

In general, these are small bits of malicious code designed to impede, cripple or destroy your computer or computer programs. This includes corrupting your operating system or deleting programs, images and files from your hard drive.

Other viruses are designed to copy themselves and send their clones via your e-mail to anyone in your e-mail list.

Still others are designed to attack Web sites by quietly using your computer, in conjunction with other infected computers, to send high volumes of page requests, overwhelming the Web site and causing it to slow down or fail. This is known as a "Denial Of Service" attack.

Most viruses and worms can be detected and eliminated with antivirus software. Installing such protection is no longer an option - you must have it if you want to use the Internet and protect sensitive information on your computer.

Phishing-Spam

These are fake e-mail messages or Web sites designed to look as though they come from people or institutions you're likely to trust: Your credit card company, financial institution, eBay, PayPal.

The messages usually suggest there's something wrong with your account or ask that you update your account information. You're then invited to click a link to a page where you are asked to provide credit card information or other personal information.

Sadly, studies by the Anti Phishing Working Group show phishers are likely to get responses from 5 percent of all message recipients - a high enough return to make the practice profitable.

Common sense is the best protection against these scams. No reputable firm, financial institution, credit card company or friend will ever send you an e-mail asking for personal or financial data - they just don't conduct business that way.

Therefore, never, never, never answer such requests. If you have a question about an e-mail, call the institution listed in the message.

Software that guards your computer system can only go so far. No firewall or antivirus program can protect you from poor judgment or bad decisions on your part.

After his run-in with the digital underworld, Bill Davis took action. He is blocking calls to 900 numbers from his phone. He installed a virus and spyware protection program. He has a firewall up and operating on his computer. And he's using a Web browser other than Internet Explorer, a favorite target for viruses, spyware and other "bad code" due to various security holes in the browser.

"Now I'm fully educated on Internet security," Davis said. "You talk about protecting your computer, but it's only when you finally get hammered that you take action."

His advice is simple: Learn from his mistakes. Take action now to protect your computer and personal information.