They are stored in the output FRS. You should be able to find the directory quite easily. It's called Filestore\Output.
The directory structure and file names below that, however, are generated by Crystal and not designed to be human-readable. Difficult, if not impossible, to tell which file is which. A third-party instance manager package may be able to do this for you. I know Apos and Quasar have products, but not sure if they can do this. There may be others.
A better way to track usage is to have your front end application track it (assuming you are not using e.portfolio) via a log file or other mechanism that you could call from ASP/CSP code when calling a report.
-Tim