unit Unit1;

interface

uses
  Windows, Messages, SysUtils, Variants, Classes, Graphics, Controls, Forms,
  Dialogs;
const
 WS_EX_LAYERED = $80000;
 LWA_COLORKEY = 1;
 LWA_ALPHA = 2;

type
  TForm1 = class(TForm)
  procedure SetTransparentForm(AHandle : THandle; AValue : byte = 0);
    procedure FormCreate(Sender: TObject);
    procedure FormConstrainedResize(Sender: TObject; var MinWidth,
      MinHeight, MaxWidth, MaxHeight: Integer);

  private
    { Private-Deklarationen }
  public
    { Public-Deklarationen }
  end;

var
  Form1: TForm1;

implementation

{$R *.dfm}

procedure TForm1.SetTransparentForm(AHandle : THandle; AValue : byte = 0);
var
 Info: TOSVersionInfo;
 SetLayeredWindowAttributes: TSetLayeredWindowAttributes;
begin
 //Check Windows version
 Info.dwOSVersionInfoSize := SizeOf(Info);
 GetVersionEx(Info);
 if (Info.dwPlatformId = VER_PLATFORM_WIN32_NT) and
 (Info.dwMajorVersion >= 5) then
   begin
     SetLayeredWindowAttributes := GetProcAddress(GetModulehandle(user32), 'SetLayeredWindowAttributes');
      if Assigned(SetLayeredWindowAttributes) then
       begin
        SetWindowLong(AHandle, GWL_EXSTYLE, GetWindowLong(AHandle, GWL_EXSTYLE) or WS_EX_LAYERED);
        //Make form transparent
        SetLayeredWindowAttributes(AHandle, 0, AValue, LWA_ALPHA);
      end;
   end;
end;

procedure TForm1.FormCreate(Sender: TObject);
begin
SetTransparentForm(handle, 125);
end;

procedure TForm1.FormConstrainedResize(Sender: TObject; var MinWidth,
  MinHeight, MaxWidth, MaxHeight: Integer);
begin
SetTransparentForm(handle, 125);
end;

end.