How much disk space is available on your C drive?  Crystal creates a lot of temp files under C:\Documents and Settings\<user id>\Local Settings\Temp.  Close Crystal, navigate to there in Explorer and delete all of the files that you can (some may be in use and cannot be deleted.)  Note:  this is a "hidden" folder so you may have to change the Folder Options (on the Tools menu) in order to be able to see it.