It depends on the version of Excel format the file is saved in. If it's in 2003 and below, it's a multiple stream binary file that will be just about impossible to decipher. If it's 2007 and above, it'll be a .ZIP format file with multiple XML files inside of it. Again, a royal pain to decipher without the SDK. Seriously, save yourself MONTHS of work and use the Visual Studio Tools for Office to do this.
A guide to posting questions on CodeProject[^]
Dave Kreskowiak