The below instruction to change comment's default font is good except that it will not change the font type. Size, background color will change, just not the defaulted type 'Tahoma'. I have tried Arial (which is my preference) and Times New Roman and others to no avail.

To change the default format of a cell Comments:

1. Press +M (the key is located between the Ctrl and Alt keys) to minimize Excel and any other open programs.
2. Right-click the desktop and select Properties from the shortcut menu.
3. Select the Appearance tab.
4. Click Advanced (skip this step if using Windows 98).
5. In the Item box, select ToolTip, and change the color.
6. In the Font box, change the font as desired, and select the font size and color.
7. Click OK to accept the new selection.
8. Click OK again at the bottom of the dialog box.

Thanks.