The Security Mentor

No matter how obscure you are, your computer might be valuable to a crook. It's kind of like how bank robbers might steal a car to use in their getaway, no matter how everyday or boring it is.

Crooks use computers like yours to send out spam, attack other computers, and worse:

Serry Winkler, a sales representative in Denver, said that she had turned off the network-security software provided by her Internet service provider because it slowed performance to a crawl on her PC, which was running Windows 98. A few months ago four sheriff’s deputies pounded on her apartment door to confiscate the PC, which they said was being used to order goods from Sears with a stolen credit card. The computer, it turned out, had been commandeered by an intruder who was using it remotely.

From a New York Times article about how bad guys use computers they've taken over.