Excel 2007 : Sharing Workbooks between PC and Mac

Hello and thanks in advance for your help.

I'm trying to set up an Excel spreadsheet that is going to be shared between a Mac and a PC over a Microsoft Server. This workbook is also password protected for read rights.

In doing some testing, I found that if I open the document on both a Mac and a PC and I make a change to the SAME row, SAME cell, and if I save using the PC first, when I go to save it using the Mac, I get a message explaining that a change was already made to that cell and it asks me which change to consider the final change. This is how it is suppose to work.

HOWEVER, If I save the file on the MAC first, I never get any message when I save using the PC. It simply overwrites whatever the Mac saved. Then If I try and save the Mac version again, I get a message saying that the File Was Not Saved.

I am using Excel 2008 on my Intel Mac 10.5.4
I am using Excel 2007 on my PC Windows XP

I saved the file as an xls since that's the format that works best with a Mac.

Can anyone help?
Thanks!

Dont think you can - there used to be some MAC software that conversted them well between platforms, forget the name now its been years since i did this

It's not really that I need the file converted (I don't think). Both computers can open the file just fine and they can both work on the file just fine, you just can't work on them at the same time and save using the Mac first.