The compatibility pack from Microsoft is designed to allow you to open the newer Excel format (.xlsx) in an older (2003) version of Excel.
All of Microsoft's documentation states that the compatibility pack should allow you to open these files in Excel 2003 after it's been installed.
Microsoft seems to suggest that you need to make sure that Office 2003 is completely updated before installing the compatibility pack: http://support.microsoft.com/kb/924074
I've also found this thread that mentioned that solution too:
I'd recommend giving that a try. It may help resolve this issue.