全てのウィンドウを最小化（API使用）

using System.Text;
using System.Runtime.InteropServices;

delegate bool EnumWindowsProc(IntPtr hWnd, IntPtr lParam);
[DllImport("user32.dll")]
static extern bool EnumWindows(EnumWindowsProc lpEnumFunc, IntPtr lParam);
[DllImport("user32.dll")]
static extern int GetWindowText(IntPtr hWnd, StringBuilder lpString, int nMaxCount);
[DllImport("user32.dll")]
static extern int GetWindowTextLength(IntPtr hWnd);

private bool EnumWin(IntPtr hWnd, IntPtr lParam)
{
    int sz = GetWindowTextLength(hWnd);
    StringBuilder sb = new StringBuilder(sz + 1);
    GetWindowText(hWnd,sb,sb.Capacity);
    if (sb.Length > 0) listBox1.Items.Add(sb.ToString());
    return true; 
}

private EnumWindowsProc lpEnumFunc = null;
private void button1_Click(object sender, System.EventArgs e)
{
    lpEnumFunc = new EnumWindowsProc(EnumWin);
    EnumWindows(lpEnumFunc,IntPtr.Zero);
}