It's the dead of night, and your home computer is on, connected to the Internet over a high-speed line. If you don't have teenagers in the house, the computer is probably not being used by anyone at this hour. But what about people outside  your house? An always-on DSL or cable-modem connection is an invitation to unwanted Internet guests--if you don't protect it properly. And even if you use a dial-up modem, those short periods of time you are on the Internet can expose you to a hostile crowd who want to either rifle through your personal data or use your computer's resources to attack a larger remote computer. Follow along as I outline the dangers and suggest some solutions to keep your computer safe.

How Net criminals find you
Every computer connected to the Internet is identified by its IP address. Whether you run your own server at home or connect to the Net via a commercial Internet service provider a permanent/temporary IP address are nonetheless assigned to you from the Internet. If you use a dial-up modem, your provider owns a block of addresses that it assigns to your connection while you're online. Malicious users, or criminal users--as opposed to hackers , the term for those who tinker with computers--know these addresses and frequently scan them looking for computers that are online. These searches can also reveal the kind of operating system you are running. Sometimes, these scans are harmless, but some may make your PC the target for later intrusion.

Stop that promiscuous e-mail
Unfortunately, scanning for IP address isn't the only way someone can track down your computer. Viruses that install code on your PC are the easiest way for malicious users to access the insides of your machine. Do you open every e-mail message sent to you? If so, you're asking for a virus. Keep in mind that an unfamiliar subject line or sender's address can signal destructive e-mail. Use your e-mail client's preview pane before you open e-mail with attachments. You might recognize the name of the person sending the mail, but the subject line, the body text, and the attached file may not make much sense, with their misspellings and odd topics. Any mismatch like this--for example, why would Grandma send me a sexy screensaver?--should alert you to a possible virus.

What do they want?
Although there is certainly a criminal element out there eager to break into your home computer to find your credit card numbers, passwords, and banking information, that element is small compared to the larger group of malicious users who want to use  your home computer for nefarious purposes. And how can they do this? Through a process called distributed computing . If your computer is a fast, modern machine, a malicious user could use some of its processing power without you even noticing a performance difference. When someone sets many computers to the same task, those machines can simulate the processing power of a single large computer.

There are legitimate distributed-computing programs out there, too; some are looking for alien intelligence and others, a cure for cancer. But malicious users have their own evil purposes. With access to hundreds of home computers, some of these people attempt to crack the strong encryption that's used to protect large corporations and government agencies. Malicious users might also use your computer as a virtual battering ram to knock a Web site off the Internet. That's called a distributed denial-of-service attack  (DDoS). Some malicious users recently blocked access to MSNBC.com with a DDoS attack.

Keep your private life private
Lastly, your Internet browser records where you've been online. It may not seem important to you, but someone else could use this information to build a profile of your interests and surfing habits. Worse, malicious users can seek out cookies that store information about you created by the sites where you've shopped or banked.