I work partly from home but regularly transfer files to my home PC from several work PCs all using the same version of Excel. I have been doing this for a number of years without problem but I recently found that files from one of the work PCs which previously opened OK won't open on the home PC and I get the message "This file is not in a recognisable format". (They have been saved in the original Excel workbook format.)
I get the same result if the file is transferred via email or USB flash drive. I thought it might be because my home PC uses Vista but I have tried them on a couple of other XP machines with the same result.