private bool mouseDown;
private Point lastLocation;

private void Form1_MouseDown(object sender, MouseEventArgs e) {
  mouseDown = true;
  lastLocation = e.Location;
}

private void Form1_MouseMove(object sender, MouseEventArgs e) {
  if (mouseDown) {
    this.Location = new Point(
      (this.Location.X - lastLocation.X) + e.X, (this.Location.Y - lastLocation.Y) + e.Y);

    this.Update();
  }
}

private void Form1_MouseUp(object sender, MouseEventArgs e) {
  mouseDown = false;
}