In Windows XP + Excel 2007 one could change the default font for comments in Excel with: right-click on desktop, properties, appearance, advanced, tooltip, enter size.
But I could not find this options sequence in Windows 7 + Excel 2010.
A similar looking dialog appears in Control Panel\Appearance and Personalization\Personalization: window color. But it has no 'advanced' button.
I already have a macro that works to change the font of all the existing comments in a worksheet.
But how does one change the default for all new comments?