The short answer is that you configure the name of an EXE or an HTTP URL to be executed when an SMS is received. Information from the SMS message is passed as parameters to the command. Optionally, the command can simply echo the reply it wants to send back to the screen (for a EXE) or return it as a text response (for an HTTP URL).
I can't help you with the bit of interfacting to an XLS file. If you can write an EXE or HTTP script to do it, then you can configure that command to run when an SMS is received.
Most of the examples you'll find on the discussion board use HTTP scripts, as that is more flexible than using an EXE. Since you're thinking EXE, here are a couple of VBS examples:
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