When you remove the border, you also get rid of the title bar and max/min buttons - you probably know that already. So, if you add a minimize button then use this code:
private const int SW_SHOWNORMAL = 1;
private const int SW_SHOWMINIMIZED = 2;
private const int SW_SHOWMAXIMIZED = 3;
[DllImport("user32.dll")]
private static extern bool ShowWindowAsync(IntPtr hWnd, int nCmdShow);
private void butMinimize_Click(object sender, EventArgs e)
{
ShowWindowAsync(Handle, SW_SHOWMINIMIZED);
}