.NET Code

using System.Runtime.InteropServices;
    public partial class FormMain : Form
    {
    	[DllImport("Gdi32.dll", EntryPoint = "CreateRoundRectRgn")]
        private static extern IntPtr CreateRoundRectRgn
        (
            int nLeftRect,     // x-coordinate of upper-left corner
            int nTopRect,      // y-coordinate of upper-left corner
            int nRightRect,    // x-coordinate of lower-right corner
            int nBottomRect,   // y-coordinate of lower-right corner
            int nWidthEllipse, // height of ellipse
            int nHeightEllipse // width of ellipse
        );
       public FormMain()
        {
            InitializeComponent();
            Region = Region.FromHrgn(CreateRoundRectRgn(0, 0, Width, Height, 15, 15));
        }
    }

    private bool MouseDownState;
    private Point lastLocation;
    public FormMain()
    {
        InitializeComponent();
    }

    private void FormMain_MouseDown(object sender, MouseEventArgs e)
    {
        MouseDownState = true;
        lastLocation = e.Location;
    }

    private void FormMain_MouseMove(object sender, MouseEventArgs e)
    {
        if (MouseDownState)
        {
            this.Location = new Point(
                (this.Location.X - lastLocation.X) + e.X, (this.Location.Y - lastLocation.Y) + e.Y);

            this.Update();
        }
    }

    private void FormMain_MouseUp(object sender, MouseEventArgs e)
    {
        MouseDownState = false;
    }

    private void PanelTitleBar_MouseDown(object sender, MouseEventArgs e)
    {
        MouseDownState = true;
        lastLocation = e.Location;
    }

    private void PanelTitleBar_MouseMove(object sender, MouseEventArgs e)
    {
        if (MouseDownState)
        {
            this.Location = new Point(
                (this.Location.X - lastLocation.X) + e.X, (this.Location.Y - lastLocation.Y) + e.Y);

            this.Update();
        }
    }

    private void PanelTitleBar_MouseUp(object sender, MouseEventArgs e)
    {
        MouseDownState = false;
    }