Hi Samir,
I am using an EXE file wich starts the main EXE, It also check for new version of the programm installed on server and some other things. In it I have the following:

IF IsRunning("Your Main application caption")
   MessageBox("Bla bla...",16,"Error")
   QUIT
ENDIF


FUNCTION IsRunning(cCaption)
     DECLARE INTEGER GetActiveWindow IN Win32API
     DECLARE INTEGER GetWindow       IN Win32API INTEGER hWnd, INTEGER nType
     DECLARE INTEGER GetWindowText   IN Win32API INTEGER hWnd, STRING @cText, INTEGER nType
     LOCAL hNext, cText
     hNext = GetActiveWindow()
     DO WHILE hNext # 0
        cText = REPLICATE(CHR(0), 80)
        GetWindowText(hNext, @cText, 80)
        IF UPPER(cCaption) == UPPER(cText)
           RETURN .t.
        ENDIF
        hNext = GetWindow(hNext, 2)
     ENDDO

RETURN .f.