Maria,
What version of XL are you using? Did you just install it? Did you
recently upgrade?
There seems to be a fair amount of discussion when you search Google
newsgroups using "file format is not valid excel" .
I am not sure what the problem is without knowing more information to match
it against prior commentary.
My suggestion is, provide more background information here, and check the
Google archives.
I am off for the evening, so hopefully others will jump in and assist.
Good luck, though I am sure you will solve it shortly.
Best regards,
Kevin