Spyware is a virus. There. I said it. Spyware loads without the user's permission, it runs invasive processes and it will eventually burn down your machine. Look around, and you will find anti-virus software vendors beginning to address the spyware problem by adding modules to their products, which, of course, cost extra since they don't consider spyware a virus. The anti-virus vendors' dictionaries are woefully inadequate, stopping less than 50 percent of the parasites. To really put a dent in the current spyware/virus problem, it would require loading three or more separate spyware clients, where each product's dictionary identifies spyware that the others had missed.
-- By Brett Arquette
http://www.eweek.com/article2/0,1759,1683485,00.asp