private   Point   mouse_offset;   
private   void   form_MouseDown(object   sender,   [color=#FF0000]EventArgs   [/color]e)   
{   

mouse_offset   =   new   Point(-e.X,   -e.Y);   
}   
private   void   form_MouseMove(object   sender,   [color=#FF0000]MouseEventArgs [/color]e)   
{   

if   (e.Button   ==   System.Windows.Forms.MouseButtons.Left)   
{   
Point   mousePos   =   Control.MousePosition;   
mousePos.Offset(mouse_offset.X,   mouse_offset.Y);   
this.Location   =   mousePos;   
}   
}

------解决方案--------------------
MouseDonw 事件是有MouseEventArgs e 参数的。你检查一下，代码写错了。

------解决方案--------------------
若是无边框窗体的移动．
  bool beginMove=false;
  int currentXPosition = 0;
  int currentYPosition = 0;
  private void Fomr_MouseMove(object sender, MouseEventArgs e)
       {
           if (beginMove)
           {
               this.Left += MousePosition.X - currentXPosition;
               this.Top += MousePosition.Y - currentYPosition;
               currentXPosition = MousePosition.X;
               currentYPosition = MousePosition.Y;
           }
       }

       private void Fomr_MouseDown(object sender, MouseEventArgs e)
       {
           beginMove = true;
           currentXPosition = MousePosition.X;
           currentYPosition = MousePosition.Y;
       }

       private void Fomr_MouseUp(object sender, MouseEventArgs e)
       {
           beginMove = false;
       }

------解决方案--------------------
呃，又是移动无边框窗体的问题，把几个鼠标按键事件处理好就行了，唯一的技巧就是坐标的计算问题，实在简单的不行了，都懒得说了。