Under Attack!:
"The largest virus outbreak in history hit millions of computers around the world this past August. Even before Microsoft Corp. and millions of victims could find a way to cope with the Blaster worm and a spate of imitators and mutations, Sobig began to live up to its name—with a vengeance.

Headline-making malware—viruses, worms, and Trojan horses—have managed to find a surprising number of unprotected PCs, despite the computer industry and media repeatedly urging people to use antivirus and firewall software. Some of the computers Sobig attacked had outdated antivirus software installed or none at all. A May 2003 study for the National Cyber Security Alliance conducted by America Online concluded that 62 percent of broadband consumers were not running up-to-date antivirus (AV) software.

But AV software alone isn't enough these days: You need a firewall, too, and privacy controls and spam filtering can further protect you. The AOL study also showed that 67 percent of broadband consumers did not have properly configured firewalls.

All manner of malware has been spreading via friendly e-mails and—more irritating—through mail no one wants in the first place—spam. Using the latest method of infection, worms send themselves out to the Internet from infected systems. Where do the worms end up? They end up in machines without firewalls or AV software. Worms either include a tiny e-mail server to send themselves out—usually with a spoofed sender address obtained from address lists—or search for unprotected shared network drives where they can unload themselves.

Once malware hits your PC, the damage can take many forms. A true virus attaches itself to a file and replicates itself when you launch the file. A Trojan horse hides on your system to do its damage, which may involve sending private data to its creator. One particularly obnoxious type of Trojan horse is a dialer, which uses your modem to call a pay number, sticking you with the bill.

A worm will often send mail to everyone on your e-mail address lists or propagate itself on shared network drives. Even viruses that don't destroy your data can wreak havoc by slowing Internet service to a crawl or hogging system resources.

Some people couldn't care less about malware and Internet security, claiming they have nothing personal or valuable stored on their hard drives. But such attitudes actually contribute to the larger problem, as these people let their machines become overrun by malware. Although you may notice only a slowdown in performance of your unprotected PC, you could actually be helping to cause massive damage on the Internet: Many viruses take part in launching denial-of-service attacks on prominent Web sites. Silently, unprotected systems in homes and offices are doing the bidding of malware that works alone or is controlled remotely by its miscreant authors, attacking other sites and systems in the process.…"

http://www.pcmag.com/article2/0,4149,1373605,00.asp