Somebody among you has opened the file with Word then later saved it. It was then saved as a Word file, not as a Excel file. That's why when you open it with Excel it will say file is not valid. What you could still do is open the file with Word, mark the whole area and copy, then open Excel, paste it and save it as a Excel file.
There is an exel file on our file server that a few of us edit (It's our calendar). A few days ago I clicked on it and got an error message reading "myfile.xls: file format is not valid" If I open word and browse to the file I'm able to open and read it. I'm running Office 2003. How is this possible? Out of the blue exel will no longer open the file but word now will.