BO and Crystal Server both have the ability to schedule based on events. One of the event types is "File". This means that it will check specific folder to see if a file exists. However, it does NOT look at the timestamp of the file, just whether or not it exists now and didn't exist the last time it checked. You can set the polling interval (how often it checks for the file.) The important part of this is that the file doesn't exist and then it does, so the file also has to be deleted at some point.
You then schedule the Crystal Report to run based on events. You have to set the time for it to run - if you want it to run multiple times throughout the day, I would select Hourly, which then gives you the option to run the report every x minutes. If you schedule with events, it will only run if the event happens and then reschedule itself for x minutes later, but only run if the event happens again.
So, if your application can delete the file, do its processing, and then create the file when processing is complete, then you can use this type of scheduling.
-Dell