How can I change the default drag and drop action on a folder in Windows 7?

You can change the default behaviour of the drag and drop with Drag'n'Drop Editor

I just tried it and works perfectly. Uninstallation of the tweak also works. My OS is Windows 7 64 SP1

http://winaero.com/comment.php?comment.news.69

I'm afraid there is no way of changing that behaviour.

If you drag & drop files without pressing any key and both locations are on the same drive, it should move it by default.

If files are on different drives, then it should copy by default.

You can continue using Shift or Ctrl as you do now, or drag & drop with your right button, which will give you the right-click menu.

Another way to do this is adding Copy to and/or Move to to the contextual menu.

Or adding options to the Send to menu. I haven't tried this one, but you can follow this tutorial.