Spyware and Viruses - What's the point?

You may have heard that people who write viruses do it for amusement or to impress their "friends". But it's more than that: It's also about power and money. Spyware is used to steal personal, business, and financial information. Viruses and other forms of malware can be used to take control of your computer to send out spam, attack other computers, and bring down Internet servers.

Here's an article at PC World that gives some insight into how networks of hacked computers, called botnets, are used.

Don't let your computer become a tool of cybercriminals! Apply Microsoft's critical updates to your operating system. Keep your antivirus and antispyware software up-to-date. Get a "second opinion" by scanning your computer for malware with proven malware fighters such as Spybot Search & Destroy, Trend Micro Housecall, or Ad-Aware. And be careful about what software you and your children download from the internet.

Further reading:
Why is my computer so slow?
Computer Security Essentials