Antivirus software is NOT optional. You must have it or you will eventually pay the price. After only one year, the Conficker worm has infected over 7 million computers. What are the odds of your computer becoming one of them? There are other botnets that control hundreds of thousands of infected machines, which can be used for spamming, launching DoS attacks, distributing child pornography or anything else the criminals want to do.

The vast majority of infections can be prevented by following a simple set of precautions:

• Use Windows Update to automatically update and fix security vulnerabilities that are discovered (you must have a valid copy of Windows for this to work).
• Download and install a recognized antivirus solution (free or paid) and make sure it is set to automatically download updates. If you have a paid version, make sure it is renewed every year. A free version is better than an outdated/expired paid product.
• Do not download and install software from unknown websites
• Do not open e-mail attachments from unknown sources and scan attachments with your antivirus program before opening them.
• If you have kids at home or novice users at the office, set them up with restricted user accounts so that they can not install software.

Considering that there are plenty of good FREE antivirus programs available, there's no excuse not to have one.