you'd be surprised. all that junk that adaware, spybot and so on try to remove, can be BOUGHT to install on your computer.
there are sites that sell instalation files for ADware and Spyware of all sorts. ofcourse half of those sites do their best to install those same programs on your pc while you're on their site, free of charge. but there must be some idiots who buy it, too.
i ran into one of these sites when i was searching for remover programs a while back.