We know a few Windows experts who insist that you should not install each Critical Update as it comes out. Instead, they say, you should wait and see whether a particular Windows patch causes problems on other people's computers before you take the risk of installing it on your own. And with all due respect to these colleagues, we say: Are you crazy? These days, the authors of viruses and worms are moving at Internet speed. The Blaster worm arrived less than four weeks after Microsoft issued an update that plugged the hole that it exploited. In theory, an attacker could launch a worm within days or even hours of a published alert.

If you see a Critical Update whose description warns that the security hole in question "could allow an attacker to compromise a computer running Microsoft Windows and gain control over it" or "could allow an attacker to execute code on the system," you should install the update immediately, without delay. We cannot imagine any inconvenience that could be worse than having your data destroyed by a virus or your computer hijacked by a hacker.

We recommend that you configure Windows' Automatic Updates feature to check for critical updates at regular intervals. You can choose to receive notifications only, download the updates automatically, or (if you have Service Pack 1 installed) have Windows update your system files automatically. To configure this feature, open the System option in Control Panel (under Performance And Maintenance if you're using Category view) and click the Automatic Updates tab. Windows XP users will find illustrated, step-by-step instructions at Microsoft's Security and Privacy Web site. [Note: The Automatic Updates feature is available for virtually all Windows versions, not just Windows XP. Ironically, you may need to connect to Windows Update to install this feature!]

2. Install and Configure a Firewall: A firewall is a system or software that controls the flow of traffic between networks and protects your computer or network from intruders. This extra layer of protection is especially important on any computer with an "always on" Internet connection, such as a DSL line or cable modem. Firewalls vary widely in their cost and features, but in general they consist of hardware, software, or a combination of the two, which prevents unauthorized users from making a connection to your computer without your permission. A personal firewall is intended to block hackers from trying to break into your PC. On networks, a firewall acts as a single point of access to the outside world, making it easier to maintain security on every network computer and to keep a log of intrusion attempts.

The built-in Internet Connection Firewall (ICF) included with Windows XP effectively blocks all incoming traffic from the outside except on ports where you've requested data. (If you request a Web page, the firewall allows that data to get through; if a hacker tries to "ping" your system, the request gets blocked.) In Windows XP, the ICF is automatically configured when you run the Network Setup Wizard. Many Windows-based programs can work seamlessly through the firewall (all traffic from the local machine is allowed out), although you might need to configure some ports manually before you can run a third-party program that uses nonstandard ports.

Third-party firewall programs (such as ZoneAlarm and Norton Internet Security) are appropriate for use with any computer running a version of Windows other than XP, and for Windows XP users who want more protection than the basic capabilities provided by ICF. In addition to intrusion detection and logging, many of these programs supply tools to help you configure traffic on a per-application basis, allow virtual private network connections, and alert you when intrusion attempts are taking place. Those extra capabilities, of course, come with a fairly steep learning curve.

Finally, if you have more than one computer using the same cable modem or DSL connection, get a hardware router, which makes it easier to share Internet access and effectively blocks unwanted traffic from outside.

3. Install and Configure Antivirus Software: Given the pandemic spread of viruses on the Internet in recent years, it's foolhardy to even think of connecting a computer to the Internet without robust, up-to-date antivirus software. Dozens of options are available, most at relatively modest prices. More important than installing the software, of course, is making sure it is capable of detecting all current viruses (using a virus signature file that is updated frequently). The best antivirus programs include software agents that handle the chore of updating signature files automatically, so you are continually protected. After installing the software and the latest updates, scan your system to ensure that you're virus-free.

Don't let a virus or worm take over your computer or network. It's important that you train all users of your computer and network (especially other family members who aren't sophisticated computer users) in safe computing procedures. Install antivirus software on every desktop computer, and configure it for regular updates, at least weekly. You should also configure your e-mail software to block or quarantine potentially dangerous attachments.

Finally, bookmark authoritative sources of information about viruses and virus hoaxes. These are some of our favorites: