To start out I found this code at http://swigartconsulting.blogs.com/tech_blender/2005/08/how_to_move_the.html:
public class Win32 { [DllImport(\"User32.
I believe that you're missing a correct WPARAM for the WM_LBUTTONDOWN message, which for the left-button is MK_LBUTTON
WPARAM
WM_LBUTTONDOWN
#define MK_LBUTTON 0x0001