You can do that with APID.
Read up on APID here:
If you are on the Mac you could use launchd.
Thanks Ralf, but I am using Windows, is there any possiblity?
I am sure there is some sort of poor () Windows equivalent but I do not know. If you do not want to use the plug-in recommended by Harbs you could try IdleTask plug-in found here: http://www.indesignscript.de/plugins.html
The advantage of APID, is that there's a fileChanged event which
automatically fires when watched files change. You don't need to do it
with idle events (which can be processor intensive).
Another option is to use Visual Basic's Timer object. I made a simple program that watches a folder in VB6 long ago, when I was making my first baby steps into the world of scripting.
Instead of using the timer, you might try something like the process monitoring described here:
(I really like "Hey Scripting Guy"--wish we had the resources to do something like it!)