When opening Microsoft Excel 2003 files in Microsoft Office Excel 2007 without having Excel 2007 already running you may find that files that a long time to open (20-60 seconds) while Excel 2007 or even Microsoft Office Word 2007, PowerPoint 2007 files take only a portion of that time to load.

The solution to this problem is easy and does not require any additional software other than the standard Microsoft Windows Explorer.

Open Windows Explorer (explorer.exe)
Select Tools | Folder Options from the menu
Go to the File Types tab
Select list item XLS (Microsoft Office Excel 97-2003 Worksheet) and click on Advanced.
If the "Appl ication used to perform action:" field the default value should be "C:\Program Files\Microsoft Office\Office12\EXCEL.EXE" /e" . Change this to "C:\Program Files\Microsoft Office\Office12\EXCEL.EXE" /e "%1" . Remember to enclose the full excel.exe path and %1 in parenthesis. Since Windows 95 this is a way to escape spaces in file names; %1 will return the file name of that file that is being opened by double-clicking.You can uncheck "U se DDE" but Microsoft Office Excel 2007 will bring it back to what you see above when you next open it.

Just confirm (hit "OK" ) on all the Microsoft Windows Explorer dialogs and next time you open a Excel 2003 file in Excel 2007 without having it already running you won’t have to wait! Problem solved

Ripped directly from m3rlin.org: http://www.m3rlin.org/wordpress/slow-opening-excel-2003-files-in-excel-2007/