most of the time it is not one ( 1 ) piece of malware , sometimes it is , but 2,3,4,5...20 "bad" programs .All trying to use the same computer .
the "best viruses / spyware" will not give the user any reason to think there box is infected . The computer will only show a small bit of extra cpu and http/ftp use .
the badly written ones ( most) will FUBAR the system .
and some now come with there OWN anti-virus to remove the competition ,so only the one virus remains .
there is BIG cash$$$$$ in bot networks
What does a hacker-kid gain by destroying someone's PC's OS,
think again ,it is not just script-kiddies anymore but organized crime( think " Russian business network")
script-kiddies do it because they are 11 to 15 years old
but running a 250,000 zombie bot net is for CASH