i have had to deal with this in programs i have made as well.
first lest address the issue
the problem is not in the shell command its in the transferspreadsheet command
the bottom line is the transferspreadsheet method cant export to the location c:\ it needs to be a place where you know anyone who uses the program will have the rights to access.
take a look at the sample database i have attached. take a look at the code and see how it builds a path specific to the logged in user. right now it is set to export to the users desktop. the only files it seems you can be sure a user has rights to is the "my documents" folder and the desktop.
hope this helps