function CompareFiles(const sFileName1, sFileName2: String): Boolean;
var
  ms1, ms2: TMemoryStream;
begin
  Result := False;
  ms1 := TMemoryStream.Create;
  try
    ms1.LoadFromFile(sFileName1);
    ms2 := TMemoryStream.Create;
    try
      ms2.LoadFromFile(sFileName2);
      If ms1.size = ms2.size Then
        Result := CompareMem(ms1.Memory, ms2.memory, ms1.size);
    finally
      FreeAndNil(ms2);
    end;
  finally
    FreeAndNil(ms1);
  end
end;

procedure TForm1.Button1Click(Sender: TObject);
begin
  if Opendialog1.Execute then
  begin
    if Opendialog2.Execute then
    begin
      if CompareFiles(Opendialog1.FileName, Opendialog2.FileName) then
        ShowMessage('Die Dateien sind gleich...')
      else
        ShowMessage('Die Dateien sind ungleich...');
    end;
  end;
end;

function CompareFiles(const SrcPath1,SrcPath2:String):Integer;
var File1,File2:TFileStream;
    lCnt:Integer;
    Buff1,Buff2:Byte;
begin
try
 try
  Result:=0
  File1:=TFileStream.Create(SrcPath1,fmOpenRead or fmShareExclusive);
  File2:=TFileStream.Create(SrcPath2,fmOpenRead or fmShareExclusive);
  if File1.Size = File2.Size then
   begin
    for lCnt:=0 to (File1.Size-1) do
     begin
      File1.ReadBuffer(Buff1,1);
      File2.ReadBuffer(Buff2,1);
      if Buff1<>Buff2 then
       begin
        Result:=1;
        Exit;
       end;
     end;
   end
                             else
   begin
    Result:=1;
    Exit;
   end;
 except
  Result:=-1;
  end;
finally
 File1.Free;
 File2.Free;
end;
end;