private Rectangle mdrRect;
        private Int32 mdrIndex = 0;
 
        void DGV_DragEnter(object sender, DragEventArgs e)
        {
            e.Effect = DragDropEffects.Move;
        }
 
        void DGV_MouseMove(object sender, MouseEventArgs e)
        {
            if (Convert.ToBoolean(Convert.ToInt32(e.Button) == Convert.ToInt32( _
              System.Windows.Forms.MouseButtons.Left)))
            {
                if (mdrRect != Rectangle.Empty && !mdrRect.Contains(e.X, e.Y))
                {
                    DataGridViewRow dgvr = DGV.Rows[mdrIndex];
                    DragDropEffects ddg = DGV.DoDragDrop(dgvr, _
                      DragDropEffects.Move);
                }
            }
        }
 
        void DGV_MouseDown(object sender, MouseEventArgs e)
        {
            mdrIndex = DGV.HitTest(e.X, e.Y).RowIndex;
 
            if (mdrIndex != -1)
            {
                Size ds = SystemInformation.DragSize;
                mdrRect = new Rectangle(new Point(e.X - Convert.ToInt16(( _
                  ds.Width / 2)), e.Y - Convert.ToInt16((ds.Height / 2))), ds);
            }
            else
            {
                mdrRect = Rectangle.Empty;
            }
        }
 
        void DGV_DragDrop(object sender, DragEventArgs e)
        {
            Point ptc = DGV.PointToClient(new Point(e.X, e.Y));
            Int32 index = DGV.HitTest(ptc.X, ptc.Y).RowIndex;
            if (e.Effect == DragDropEffects.Move)
            {
                DataGridViewRow RowToMove = e.Data.GetData(typeof( _
                  DataGridViewRow)) as DataGridViewRow;
                DGV.Rows.RemoveAt(mdrIndex);
                DGV.Rows.Insert(index, RowToMove);
                DGV.Rows[index].Selected = true;
 
                for (int i = 0; i < DGV.Rows.Count; i++)
                {
                    if (i != index)
                    {
                        DGV.Rows.Selected = false;
                    }
                }
            }
        }

Gruß
Christoph