To protect your own computer from damage, and to prevent your computer from being used to pass on infections to other machines without your knowledge. Many people are reluctant to shell out the cash for decent antivirus software. I'll agree that at roughly $50 for a decent antivirus program, it isn't exactly small change.

The top antivirus software programs cost as little as 2 months internet connection. They all include constant and unlimited automatic updates that keep your system safe and clean. If you can afford $1500+ on a computer, $300+/year on internet connections, you can definitely afford another $50 once to make it safe (and prevent it from being used to spread viruses and Trojan horses behind your back).