First off I have read the related posts here as well as the below link but
I'm not 100% convinced it solves my issue.

http://support.microsoft.com/default.aspx?scid=213904

My issue that I have is that I use Excel 2007 on my main PC but some of the
managers at my work still use 2003 (business default). In order for them to
read the files I save it as an XLS (instead of the XLSX). I don't have too
many formats in the file (some time, percentage and whole number formats, and
a header that is color), but whenever I open the file in 2003 (on my 2nd PC)
it gives that error but then allows the file to open but strips all
formatting away first. My file has multiple tabs and subtotals which is also
stripped away on the lefthand side, but you can turn off subtotals and remove
the lines that are inserted by excel. This is quite a chore to reformat
everytime and I'm almost ready to ditch 2007 and stick with 2003 since its
the business standard.

Aside from saving all my equations to Notepad and re-inserting into excel
2003, does anyone know another way around this? My 2003 PC has the plug in
for 2007 and I've tried saving as XLS and XLSX and I get the issue both ways.