Unfortunately, Microsoft doesn't provide a direct way to download these fonts without buying Windows Vista.
But here's a simple trick to download the Vista fonts for free and legally without buying a Vista license:
Download either the free Microsoft Powerpoint 2007 Viewer or the Microsoft Office Compatibility pack - both the software include the new Windows Vista fonts.
Once you install the above Microsoft programs, the Vista Fonts also become available for use on your Windows XP system. The fonts are Candara, Consolas, Calibri, Cambria, Constantia and Corbel. Thanks GlobalNerdy.
Related: Download Windows Vista Backgrounds