I have for instance installed nothing special on this laptop. You install Outlook 2010, it creates a folder in Documents. OneNote, too. The AV-converter I use, video editor, Visual Studio, Windows Live Writer for blog posts, Nokia Ovi Suite and so on.
I think I have never seen a Windows computer (XP or later) where when the computer has been used a bit longer period at least one third of folders in Documents wouldn't be created by installed applications. I'm so sure about this I could make a bet.
Just checked Angie's laptop over her shoulder while she's once again tagging her photos: 22 folders in her Documents folder created by installed applications. And she really has only the basic apps installed on her laptop, Office, Ovi and some image and video editing / manipulating apps.
Kari