How much data are you pulling? What type of database are you connecting to? Are you using any formulas in the selection criteria or sort/group that would cause Crystal to pull everything into memory before filtering/sorting instead of pushing that task to the database?
Crystal does a LOT of swapping to disk when it is processing a report and it's not always very good about cleaning up after itself. You need to go to your temp folder and delete all of the files that start with "~" - these are Crystal's temp files.
-Dell