Associating .xls files with Excel 2003 when Excel 2007 is installed

I have both Excel 2003 and Excel 2007 installed in Win XP.

I want to continue to have all Excel .xls files be associated with Excel
2003 for opening by double-click on the file, and have only the other
Excel-2007-specific files, like .xlsx, associated with Excel 2007, opening
in Excel 2007 by double click.

But all of them insist on opening in Excel 2007. How do I stop this?

The Windows XP Folder Options / Open With.. method won't let me change
the file association for .xls back to Excel11. It refuses to add Excel11 back
in as a choice after I browse to it.

How do I edit the Registry to overcome this -- for Excel and for all the
other Office 2007 programs (Word, PowerPoint, Access)?