This problem happens on microsoft Vista.
I'm using notepad++ v4.0.2 and when I want to move any docking pane and begin to drag it causes an informative dialog to "appear" under (behind) the notepad++ window with Title "SetWindowsHookEx(KEYBOARD) failed" and contains the message GetLastError() returned 1428 and an ok button.
While the dialog is there, the mouse is unresponsive.
Once you press ALT-TAB and hit enter you can move the mouse and a box slowly redraws making very dificult to reposition the panel.
To see the error dialog, make the notepad++ window half size your screen and position it to the left, make a panel visible, try to drag it and the system sound associated to error notices plays, If you check your taskbar, the error window its there.
Press ALT-TAB to change to that error window, then hit ALT-Space to get the window options popup menu, select move (with cursor keys and clickit), and using cursor keys move the window to the right to bring it to a visible area to see it.