Fortify your PC against intruders

Threats to your computer run from the merely annoying, such as pop-up ads, to the truly insidious, such as viruses, worms and spyware that can rip off financial information stored on your personal computer. These predators often arrive as hitchhikers when you download files, inadvertently click on a link in spam or OK a license agreement that contains a buried notice of tag-along software.

In fact, 90 percent of home broadband users already have some type of troublemaker on their PCs, according to a study by the National Cyber Security Alliance. But those with dial-up connections need to fortify their PCs, too.

• Virus killers. These are software programs that detect and destroy viruses and worms — destructive pieces of code delivered via e-mail. Two first-class products are Norton Internet Security 2004 (free trial, then $70, www.symantec.com) and McAfee Internet Security Suite 6.0 ($70, us.mcafee.com). Although Norton and McAfee include firewalls, spam fighters and additional privacy features, these programs are not one-stop solutions. You may need to fill some gaps by enlisting additional security programs.

• Personal firewalls. These are traffic cops that look at what enters and leaves your computer. ZoneAlarm (free, www.zonelabs.com) is a good, basic firewall that can be updated online at no cost to battle the latest generation of pesky intruders.

• Counter-spyware. Spyware is the most insidious type of software. Culprits include keyloggers, which track the keys you press to pick up passwords, credit-card numbers and banking information; dialers, which take over your modem to call long-distance phone numbers; Trojan horses, which surreptitiously take over parts of a PC to carry out their own instructions, such as tracking your surfing activity; and browser hijackers, which reset your browser home page to a site (often packed with porn links) designed to drive business to the hijacker.

Spybot Search & Destroy 1.2, free, available at CNET's www.download.com scans for known spyware and removes it, and blocks new spyware from getting in. It can be updated free online.

• Pop-up-ad blockers. Ad-aware 6.0 (free, www.download.com) eliminates most pop-up ads, as well as spyware, though it sometimes misses spyware caught by Spybot and vice versa. Google, www.google.com, also offers free pop-up-ad-blocking software.

• Browser protectors. Antivirus and firewall programs don't monitor browsers because browsers have carte blanche to communicate with the Internet. So intruders can still get in, especially if you use Internet Explorer. Secure IE 2004 Suite, $50, www.winferno.com, is a package of browser-security controls that determine what Web sites can do to your computer.

Spyware is the most insidious type of software. Culprits include keyloggers, which track the keys you press to pick up passwords, credit-card numbers and banking information.