I recently found a a piece of parasite software on my system, thanks to the terrible security of Windows. :-P Fortunately I am a fairly savvy conputer user, knowing a fair bit about the internals of Windows so I could remove this annoying piece of *#!!* software — though it was damn tricky finding every place it hid itself and its data. I'm still not sure I zapped all of it. :-P
Parasite software is usually spyware. It scans your harddrive for personal information (address, credit card numbers, etc...) and keeps records of every site you visit, sending that data somewhere, usually to some company who will sell that info to third parties. Parasite software doesn't show up in the Add/Remove programmes control panel which means to uninstall it you have to know what files it secretly installed in the first place, and even if you can hunt down and find all those well hidden files, if you don't get every last one they can redownload and reinstall themselves the next time you go online. This is scary stuff - programmes that can't be deleted that spy on everything you're doing on your computer and send the info God knows where!
So I did a little bit of research in to parasite-ware and spyware and I found two webpages which are very useful. The first one (http://www.doxdesk.com/parasite/) has a little script on it to detect whether your computer has any of about ~100 different pieces of spyware as well as instructions on how to get rid of any that are found. The second (http://www.unwantedlinks.com/spyware-info.htm) has a good explanation (even for non-geeks) of what parasite spyware does and a link to a list of about 800 programmes that secretly install this crap.
I recommend that everybody who uses Windows on their computer visit that page (with Internet Explorer, not Netscape, Opera or a different browser) because these parasitical spyware programmes are NOT not detected by anti-virus software! The annoying parasites are usually secretly bundled in with free softare people download from the next (Grokster, BonziBuddy, Kazaa, LimeWire, AudioGalaxy, CuteFTP are some examples of the more popular ones), but some webpages secretly install parasite software when you surf to them.
Also, a very popular program called Gator, which is advertised a personal utility program that helps you manage personal information (name, birthday, address), bank account information, credit card numbers, passwords and so forth. It's major feature is to automatically fill out forms on web pages with that personal info — which sounds cool ("hey, less typing!"), untill you realize that Gator also sends that info of yours back to it's creators for them to sell to third parties! Not only that, but Gator is also "theftware", which means that when you are surfing, Gator scans all the web pages you visit for ads and switches them for ads of Gator's clients. On the surface this doens't sound too bad - and ad is an ad, right? Wrong! Most of those webpages have those adds to generate revenue so the people can keep their pages online. But since Gator is secretly replacing those ads with ones from Gator clients, the people don't get the revenue they deserve. Gator is extremely unethical.