private Point lastLocation;
private Point lastMsPoint;
private Size lastSize;
private void label1_MouseDown(object sender, MouseEventArgs e)
{
    this.lastLocation = this.label1.Location;
    this.lastSize = this.label1.Size;
    this.lastMsPoint = Control.MousePosition;
}

private void label1_MouseMove(object sender, MouseEventArgs e)
{
    if (e.Button == MouseButtons.Left)
    {
        Point mp = Control.MousePosition;
        if ((ModifierKeys & Keys.Control) == Keys.Control)
        {
            this.label1.Size = new Size(this.lastSize.Width + mp.X - this.lastMsPoint.X, this.lastSize.Height + mp.Y - this.lastMsPoint.Y);
        }
        else
        {
            this.label1.Location = new Point(this.lastLocation.X + mp.X - this.lastMsPoint.X, this.lastLocation.Y + mp.Y - this.lastMsPoint.Y);
        }
    }
}