My work is to fix computers and usually it is indeed the user that is to blame from having spyware on his machine, using IE and going to sites that has lot of ads usually brings shit in.
Then there is this other group, they don't have much spyware, but more viruses, their antivirus is outdated or they have none, viruses have come usually from e-mail, they have used usually outlook express.
I have not had a virus, but Avast! has few times blocked spyware from ad, even I do use firefox and adblock, drivingitalia.it was where it stopped spyware last time.
Avast is good, it is not resource hog and updates are very quick and don't disturb my working.