On two computers with Windows 7 at work and at home with the same version of iTunes (10.1.0.56) at work I can drag-and-drop files into iPod, and I can not at home. At home only adding them first through a dialog box to a Library and only after that drag-and-dropping into iPod (or iPad) works.
Initially I thought the problem is in iTunes, but looks like it's the same version, so most likely it's the Windows 7 config parameters.
It's 64-bit W7, and I have admin privileges in both cases. The only difference I could think of is it's an enterprise version at work and ultimate at home.
What should I change at home to enable that drag-and-drop-ing files into the application?