Uses Vcl.Forms, Vcl.SvcMgr....
......

begin
  if ..... если в GUI режиме....... then
  begin
    Vcl.Forms.Application.Initialize;
    Vcl.Forms.Application.MainFormOnTaskbar := True;
    Vcl.Forms.Application.CreateForm(TfmMain, fmMain);
    Vcl.Forms.Application.Run;
  end else
  begin
    if not VCL.SvcMgr.Application.DelayInitialize or VCL.SvcMgr.Application.Installing
      then VCL.SvcMgr.Application.Initialize;
    VCL.SvcMgr.Application.CreateForm(TRLService, RLService);
    VCL.SvcMgr.Application.Run;
  end;
end;

if StartServiceCtrlDispatcherW(@ServiceTable)=0 then
  begin
   err = GetLastError();
   if err = ERROR_FAILED_SERVICE_CONTROLLER_CONNECT then // not run from service manager
    begin
     Application.Initialize;
     Application.CreateForm(TFrmMain, FrmMain);
     Application.Run;
    end
   else
     WriteLog('Service initialization error ', err);
  end