I mean, i usually have the common sense on what is safe and what isnt but occasionally i do visit dodgy sites (not pr0n) and have been known to get a few nasties - some harder to remove than others. My sister is the main culprit - especially to those messenger spread stuff and also via bebo etc. etc.
Sometimes you dont know you have spyware until you scan though
Superantispyware takes a long time to scan but it found an adware (vundo variant) whereas spyware doctor completely missed it while picking up 60 odd other pieces (most were cookies)
I use the Spybot Search and Destroy immunize feature, I never get anything but tracking cookies. I don't know how well it works though cause I stay away from shoddy sites and I am careful about what I click.
turns out that Avast is pretty good laxative as well.
On my own in the house last Thursday, all PCs on, and nearly sh@t myself when I heard voices in another room - 'twas Avast telling me that it had updated itself.