Excel 2007 uses the very nice Calibri font by default in Windows Vista, but if you'd like to use a different font or change the default size, you can easily adjust the setting in the Excel options screen.

First click on the Office Button and choose Excel Options in the lower right hand corner of the resulting menu.

Select the Popular tab on the left, and then in the middle of this window you can change the default font using the selection.

Now when you create a new worksheet, you'll see that the font is set automatically to your new selection.

You can always change the font using the Ribbon as well, but it's only active for the current worksheet.