data link cannot be created because Excel cannot be found

I'm using AutoCad 2013, with Excel 2013 on a 64-bit machine.  I've been able to datalink Excel files to previous drawings, but recently received this error in new drawings:  The data link cannot be created because Excel cannot be found.

Excel is open and running on the machine.

I've tried restarting AutoCad, Excel, and the machine, with no success.

I'd appreciate any suggestions, short of re-installing either MS Office or Autocad.

Thanks. 

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