Many people seem to run into this problem in both 2003 and 2007. In Excel
2007 most often the issue is related to the exessive number of unused often
corrupted styles and not so much cell unique cell format combos (hard to
imagine someone can actually hit over 64,354 combos). I wrote
a utility to fix XL2007 OOXML files that can also be saved down to XL2003
after the clean up.
Here is the link to the blog post:
http://sergeig888.spaces.live.com/bl...9444!534.entry

Requres .Net3.5 and MS Excel 2007. Will fix xlsx or xlsm files. Won't work
on binary xlsb or password protected files (read-only passwords are OK).

No need to run the risk of further corrupting your file by using Open Office
like some people recommend in other Excel forums. Utility is free and comes
as is.