[NumLock]

файл проекта:

Код:

program Project1;

uses
  Windows, Messages,
  Forms,
  Unit1 in 'Unit1.pas' {Form1};

{$R *.res}

var
  PreviousInstanceWindow : LongWord;

begin
  PreviousInstanceWindow := FindWindow('TfMain',nil);
  if PreviousInstanceWindow <> 0 then begin
//    if IsIconic(PreviousInstanceWindow) then
      SendMessage(PreviousInstanceWindow, WM_USER, 0, 0);
//    else
//      SetForegroundWindow(PreviousInstanceWindow);
    Halt;
  end;
  Application.Initialize;
  Application.CreateForm(TForm1, Form1);
  Application.Run;
end.

главная форма:

Код:

unit Unit1;

interface

uses
  Windows, Messages, SysUtils, Variants, Classes, Graphics, Controls, Forms,
  Dialogs;

type
  TForm1 = class(TForm)
  private
    { Private declarations }
    procedure WMUSER(var Message: TMessage); message WM_USER;
  public
    { Public declarations }
  end;

var
  Form1: TForm1;

implementation

{$R *.dfm}

function SetForegroundWindow(AHandle: THandle): Boolean;
var
  AWindow: THandle;
  AThreadId: Cardinal;
  AProcessId: Cardinal;
begin
  AWindow:=GetForegroundWindow;
  AThreadId:=GetCurrentThreadId;
  AProcessId:=GetWindowThreadProcessId(AWindow, nil);
  AttachThreadInput(AThreadId, AProcessId, True);
  Result:=Windows.SetForegroundWindow(AHandle);
  AttachThreadInput(AThreadId, AProcessId, False);
end;

{ TForm1 }

procedure TForm1.WMUSER(var Message: TMessage);
begin
  if not Visible then Show;
  ShowWindow(Application.Handle, SW_SHOW);
  Application.Restore;
  Application.BringToFront;
//  SetForegroundWindow(Handle);
end;

end.

только WM_USER+что-то, имхо