Go to practically any game cheat-code site and you're likely to be assaulted with at least a few download attempts. Just let 'em go through instead of insisting "no," and you'll have your first set of fleas. Make sure to allow all activeX for maximum effect. You might have to try a few...some seem to have cleaned up their acts lately.
Of course, the official sites of many parasites usually offer their "wonderful" software directly. If you get it there, though, that is definitely an opt-in, which means you won't get the side-benefit of being able to claim that someone snuck it onto the computer.
Download some free screen savers or wallpapers (try whoever ranks well in Google--people bother with SEO the most when there's money involved. If the #1 doesn't get you an infestation, then grab some from each of the top 10--there's bound to be some parasites in with some of that "free" download stuff.) Same with "free" smilies for your email! Might as well get something remotely cool, along with your parasites... Note: You may have to install the screensavers, wallpapers, etc., rather than merely "downloading" them, in order to unleash the infestation.
File-sharing software is another type of download that's known for coming with unwanted extras.
There is no knowledge that is not power. ~Hemingway