That should be all you need to do. the .xlsx files don't even have code in
them (as .xlsm would indicate), so they're "just" an Excel workbook that
should open if you have the compatibility tool installed.
You might try uninstalling the compatibility tool (it should be in the
Add/Remove software list) and then reinstalling it. Make sure your copy of
Office/Excel has the latest patchs, and has SP3 installed also.
Sometimes Office 2007 created files take longer to open than 2003 files, so
you might also try opening the file again and then going out for dinner and
seeing if it's opened up when you get back - seriously; one user reported a
shared Excel file that took 15 seconds to open in Excel 2003 taking 45
minutes to open in Excel 2007 (it was an Excel 2003 file, not Excel 2007).
You may have a reverse of that situation happening here.
