private void Form1_MouseDown(object sender, System.Windows.Forms.MouseEventArgs e)
{
if(e.Button == MouseButtons.Left)
{
mouseDown = true;
}
}

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

private void Form1_MouseMove(object sender, System.Windows.Forms.MouseEventArgs e)
{
if(e.Button == MouseButtons.Left)
{
if(mouseDown)
{
this.Location = new Point(MousePosition.X,MousePosition.Y);
}
}
}