The very fact that you are connected to a public network along with millions of others means that your PC is vulnerable. It is open to receiving thousands of little software programs that are specifically designed to either harm, disrupt, or exploit your computer.

** Protecting your computer against malicious intruders

Any time you receive a file from some other source on the web, there is a chance that file will be "infected" with a virus, or will contain an attached bit of adware or spyware. That applies to viewing web pages, and receiving email messages. It also applies to downloading image, audio, and video files.

Email attachments are especially notorious for harboring viruses and malware. You receive a message that has an attachment beckoning you to double-click on it. When you do so, it installs a little program that resides somewhere deep within your PC, ready to do whatever it has been programmed to do -- whether that is spying and reporting on your computer habits, hijacking your home page and redirecting you to some other site, or popping up aggravating ads for gambling or pornography sites.

Viruses go well beyond that. They are intentionally designed to run programs that damage important operating system files, and usually have some way of exploiting your email program to send the infected message to hundreds of others in your contact list. That is why they are called "viruses" -- because they can spread like wildfire to thousands and even millions of computers.

** Antivirus programs

Antivirus programs like Norton Antivirus 2006 were designed to do just what their name implies -- to guard against viruses. They do this by using a four part process. First when the anti-virus program is installed it "cleans" the system. Every file on your hard drive is scanned to see if it is infected. When an infected file is found it is either "quarantined" or deleted.

Second, the antivirus program stands guard and automatically scans any new files, email messages or web page files that are downloaded or copied to your computer's memory. If any of these files are found to be infected, the antivirus program blocks them from being copied.

Third, the antivirus program runs periodic scans -- usually every few days -- of your entire file system to make sure no infected files have made it past the automatic scan.

And fourth, the antivirus program has a system for being automatically updated. Since new viruses are being created every day, it is important to update your system of protection every day. Usually this is done by automatically downloading and installing updates to the virus descriptions.

** Guarding against spyware

Spyware is a relatively new problem confronted by the PC owner. The point of spyware is to monitor your keystroke patterns and use this information for some commercial or advertising purpose. The spyware program is usually installed along with some freeware such as a music or video sharing program. The user does not consent to the installation and is often not even aware of it.

Some spyware programs are relatively harmless, but others can have a dramatic impact on your computer's performance -- even bogging it down completely.

Newer versions of an antivirus program such as Norton Antivirus 2006 detect and remove spyware, keystroke loggers and other unwanted monitoring software. They also come with regular updates to stop new spyware from infecting your computer.