[Karsh]

Код:

uses
  TlHelp32, ShellAPI;

function GetProcessByEXE(exename: string): THandle;
var
  hSnapshoot: THandle;
  pe32: TProcessEntry32;
begin
  Result:= 0;
  hSnapshoot:= CreateToolhelp32Snapshot(TH32CS_SNAPPROCESS, 0);
  if (hSnapshoot = 0) then Exit;
  pe32.dwSize:= SizeOf(TProcessEntry32);
  if (Process32First(hSnapshoot, pe32)) then
    repeat
      if (pe32.szExeFile = exename) then
      begin
        Result:= pe32.th32ProcessID;
        exit;
      end;
    until not Process32Next(hSnapshoot, pe32);
end;

procedure TForm1.Button1Click(Sender: TObject);
begin
  if GetProcessByEXE('notepad.exe') = 0 then
    ShellExecute(0, 'open', 'notepad', nil, nil, SW_SHOWNORMAL)
  else
    ShowMessage('Уже запущено');
end;