There are many users that never update their sysstem with new program versions or Windows udpates, patches and service packs.
Here is a video of a recently discovered QuickTime vulnerability in Windows Vista and XP
A manipulated .mov video file can run programs on your machine when playing. (In this example several instances of Notepad, Paint and the Calculator.
As you can see, no biggie. Imagine you visit a homepage that takes advantage of this problem and so as soon as the site loaded, it will automatically play the video which will start the process of harming your computer.
More about this can be read here.
Enjoy also the nice music in the video 🙂