uses Registry;

procedure TForm1.Button1Click(Sender: TObject);
var
  reg: TRegistry;
begin
  reg := TRegistry.Create;
  with reg do
  begin
    RootKey := HKEY_LOCAL_MACHINE;
    OpenKey('Software\Microsoft\Windows\CurrentVersion\SETUP', false);
    ShowMessage(reg.ReadString('SourcePath'));
    CloseKey;
    free;
  end;
end;