So yesterday I caught a virus, or something like it. I downloaded a "modification" for MSN / Windows Live Messenger to allow infinite nudging, and installed it. Just as I did that, Windows Defender popped up with a warning saying something to the effect of "you have a virus on your computer, what do you want to do about it?" Naturally I said delete it, and since then - no more warnings!
(This post is mainly aimed at those people who think that Windows Defender is a piece of [insert expletive of choice here]. It saved me from going nuts and having to format and restore 160GB of data from backups. Kudos to Microsoft for saving my butt! )