I've had some luck in circumstances like this with opening the file in OpenOffice.org's spreadsheet module, making some trivial change, and saving it again as an Excel format file. If your file is truly corrupted, this might not work, but if the damage is minor or stems from Microsoft's inter-version compatibility issues, O😵 seems to be more robust than Excel about handling errors.
I've found O😵 to be good at repairing Word files, as well.
You could try other nominally-Office-sompatible programs for this as well; I think AppleWorks can open or import Excel files, and I know tha ThinkFree Office can.
Good luck.
Crikey