Compatibility Issues between Mac and PC

Version: 2004
Operating System: Mac OS X 10.5 (Leopard)

I just purchased a Mac for my home business. I was formerly using a PC
w/windows vista. My problem lies in that I need to send other co-workers
excel documents. I have no problem opening their documents, but they keep
having issues with mine. I know none of them have 2008. I am unsure of
their exact versions, but I do know that they all have pc's w/windows. The
issues that they are having range from them not being able to save after
making changes to the document - to - not just being able to open it at
all from their email!! I have tried to save the file types as -

Excel workbook
Excel 97 - 2004 & 5.5/95 workbook.

And lastly there are macros from amazon.com on the all of files.

PLEASE HELP!! I don't want to go back to my PC!!!