I'm having the same problem
I can use Excel on the computer, if it originated on this computer.
If I go to Excel, open new document, I can create a new document, and open it and edit it
But, if I try to open an excel file that has been emailed to me, or from a disk (that originated on another comptuer), it says the .xls file is not valid.
I can open the file on other computers, but not this one....??????