If the default font that Excel 2003 uses for all new worksheets doesn’t suit your needs – change it by selecting Tools > Options > General tab and set the Standard font and Size to your preferred choice and choose Ok.
In future, all new workbooks you create will be set by default to this font although those you have previously created will remain unchanged.
If you’re using Excel 2007 and you don’t fancy the new Calibri font, click the Office button, choose Excel Options and click the Popular group. From the Use this font dropdown list choose the font to use for your new worksheets and click Ok. You’ll need to close Excel and restart it for the new font change to be in force.