MyDoom

The newest version of MyDoom does not even have a file attached to it, making it look even less suspect, and less worm- or virus-like.

Instead, it exploits one of the more recently discovered holes in Internet Explorer, through which clicking on a URL can cause a file to be downloaded to the unsuspecting user’s computer. Once downloaded to the user’s machine, the file executes, harvests email addresses, and starts spewing spam from the user’s computer.