>In this case, will have to have a class that uses a timer, that periodically checks for the existence of a file at a specific location.
>
>And do I have to have an instance of the class on every from in my application? I am just trying to think how I can work on this. Will Form - Activate be a good place to have this code?

No need for all that. You should only have 1 timer for this. YOu can establish it in your MAIN prg with you global application object [goApp]:

goApp = NEWOBJECT('custom')  && instanciate your goApp object here
goApp.AddProperty('oAppTimer')
goApp.oAppTimer = NEWOBJECT('_AppTimer', 'AppTimer.prg')
goApp.oAppTimer.Release()

**** Code for the AppTimer.PRG file ****

DEFINE CLASS _AppTimer AS Timer

	PROCEDURE Init
		THIS.Interval=300000  && this is equal to 5 minutes
		THIS.Enabled=.t.
	ENDPROC
	PROCEDURE Release
		RELEASE THIS
	ENDPROC
	PROCEDURE Timer
		WAIT WINDOW 'Fired...' TIMEOUT 1
		*!* put your file check code here and remove wait window
	ENDPROC
ENDDEFINE