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First of all, you should find handle of the window, which are the desktop. And after that, you may work with it like with any other window.

// Hide icons on the Desktop
procedure TForm1.Button2Click(Sender: TObject);
begin
  ShowWindow(DesktopListViewHandle, SW_HIDE);
end;

procedure TForm1.FormCreate(Sender: TObject);
begin
  DesktopListViewHandle:=FindWindow('ProgMan', nil);
  DesktopListViewHandle:=GetWindow(DesktopListViewHandle, GW_CHILD);
  DesktopListViewHandle:=GetWindow(DesktopListViewHandle, GW_CHILD);
end;

// Show icons on the Desktop
procedure TForm1.Button1Click(Sender: TObject);
begin
  ShowWindow(DesktopListViewHandle, SW_SHOW);
end;