Remember at the end of my first post on Word 2007, I promised some information on Excel 2007. This is it.
I had the same kind of issue with Excel 2007 that I'd had with Word 2007. That is the default formatting uses the Calibri font that users without Office 2007 probably won't have.
Fortunately, it's easier to solve in Excel than in Word.
Just click on the Office button, click on "Excel Options" near the bottom right and choose "Popular."
Go down to the "When creating new workbooks" section and find the "Use this font" drop-down box. Change this to Arial and click Ok.
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Many thanks to Helen Bradley for setting me straight.