Blizzard, makers of World of Warcraft, have deployed spyware to catch "cheaters." If you want to avoid the spyware, you can install Sony's rootkit DRM (just load a store-bought CD with Sony's DRM on it) and then use its cloaking capabilities to hide your WoW app:
World of Warcraft hackers have confirmed that the hiding capabilities of Sony BMG's content protection software can make tools made for cheating in the online world impossible to detect. The software–deemed a "rootkit" by many security experts–is shipped with tens of thousands of the record company's music titles.
Blizzard Entertainment, the maker of World of Warcraft, has created a controversial program that detects cheaters by scanning the processes that are running at the time the game is played. Called the Warden, the anti-cheating program cannot detect any files that are hidden with Sony BMG's content protection, which only requires that the hacker add the prefix "$sys$" to file names.