Excel xls files open as Machine Code
 

Recently I created 11 Spreadsheets with Office 2003 on Machine 1. Using a USB
Drive I copied all 11 files to Machine 2. On Machine 2, with Office 2003, 7
files open correctly with data in tact. The remaining 4 files open with what
appears to be Machine Code, which is unreadable. The files extensions are
..xls in all cases.
Is there any way of converting these files back to readable data or
recovering the data by some means.
I have never experienced this before and do not know how it could happen to
only some files of the set.
Any assistance will be most appreciated.
Thank you.