The two most popular antivirus programs have already been mentioned but here they are again:

• Symantec Norton Antivirus: This one is very popular and a best-seller.
• McAfee Virus Scan: This is also an excellent program, and some people like it better than Norton Antivirus.

Both websites offer downloadable, free trial versions. You need one of these two programs or some other antivirus software! If they ask you to pay for a subscription to download the updates -- do it. It's worth it.

If you're a student or a school employee, check with your school to find out whether a site license has been purchased for an antivirus program. If so, you may be entitled to a free copy of an antivirus program, along with free updates for as long as you remain a student or employee there. In addition, some cable modem companies and DSL (Digital Subscriber Line) companies offer free virus software as part of your subscription.

Getting Virus Definition Updates

Most antivirus programs can't detect viruses that they don't know about. There are exceptions, such as programs that monitor the file sizes and dates of essential system files and warn you if they're about to be changed. However, the vast majority of threats circulating today are not true viruses in that they do not actively infect your existing .exe files or boot-sector. Instead, they're Trojan horses, backdoor programs, or worms, which don't usually have behaviors that trigger that kind of proactive detection. This means that updated definition files are your only reliable line of defense against new virus threats.

Programs such as Norton Antivirus and McAfee Virus Scan include automatic updating that checks for new definitions on the company's server and installs them automatically. For example, as shown in Figure 4-3, the Norton Antivirus Corporate Edition software has a LiveUpdate button you can click.

Figure 4-3: LiveUpdate keeps Norton Antivirus up-to-date.

Be warned, however, that some services, such as Symantec's Live Update, update their servers only once a week except during peak periods of virus problems, so you might not always get the latest updates by running Live Update (or whatever the auto-updater is for your software). Going manually to the company's website and comparing the date of the most recently posted definitions to the date shown in your software is one way to ensure you have the latest updates, but that can get exhausting.

Some people will call you paranoid if you download virus definition updates every single day. Don't go overboard with this.