Spyware, are programs that are put on your computer and you often do not know about it. You can get spyware, from say downloading a game, and the game maker also installs a tool that can do other things.
Cnet’s John Borland, intentionally installed spyware and the results were not so nice. He writes For weeks I had been investigating companies that claimed to offer spyware-fighting software, but secretly distribute spyware or other advertising apps along with their products. Because I wanted to put myself in the shoes of the average computer user, I deliberately let down my protections to load the applications in question.
Not the swiftest decision, I was told by the IT technician who hauled away my machine to reimage the hard drive. After weeks of increasingly worse headaches, the machine started surreptitiously pumping viruses onto the corporate network. (full story)