In Excel 2003, the justify button isn't displayed by default in formating toolbar.
Howerever you can add this button (or many usefull buttons) mannually.
Go to Tools>Customize, and choose Format from the Categories , and scroll down to Justify.
(from rightside panel). Drag the button up onto your Formatting toolbar.
I don't have Excel 2007 but you should get this button from the customization options in Excel-2007.
Or you can use excel-2003 classic menu addon in Excel - 2007 from below page by shailesh.
https://shahshaileshs.web.officelive.com/Exshail_Classic_Menu.aspx
It has defult justifying button in the menu as per below image from his website.

Kaushik