Hi:
You may have other issues in your Stored Process, however. Your Stored Process will run on the Stored Process server or the Workspace Server. If your Excel spreadsheet is down on your C: drive (or on a user's C: drive), it is unlikely that your servers will have visibility of the Workbook. Generally speaking, the servers in the BI platform do not have access to a user's local drive.
It is possible that you might reference the Excel file by UNC name (\\server\share\file_path ) however, it might be easier to upload the Workbook to a location on a server known to the SP server or the WS server. It may be possible to use EG tasks to upload to the servers (I can't remember whether that functionality is already built-in or is one of the upcoming features.)
You might want to check with Tech Support for more help with this task.
cynthia