Excel 2003 and Excel 2007

Hi,
I work with Office 2003 and intstalled on the same computer but in a different folder only Excel 2007. Now my Excel files open with Excel 2007 by default and I would like to change it back to 2003 and to use Excel 2007 only when I decide to. I’ve tried to solve the problem through Folder Options/File Types/Change “Opens with” but nothing hapenned.
Can you help?
Thank you in advance.
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In the meantime I got help in the MS Discussion group by Dave Peterson: unregister Excel2003 and register it again and it worked.