Network World, one of the leading paper magazines for network adminstrators has a nice article on Spyware in the January 26, 2004 issue:

http://www.nwfusion.com/research/2004/0126spy.html

Spyware programs such as Cydoor, Gator, Lop.com and Xupiter install without the user's knowledge by piggybacking on peer-to-peer file-sharing programs, cute executable images or a long list of freeware.....