How can I email a font from Vista to Windows XP?

I'm not sure I understand your question. You can e-mail files, and fonts are in files. Is that what you want?

Windows Vista comes with a number of fonts built-in. You can also download additional fonts into Vista from

Download Windows Vista Fonts from Microsoft Office Website - Digital Inspiration

You can download additional fonts for Windows XP from

http://www.styopkin.com/free_downloadable_fonts.html

If there's a Windows Vista font that you want to send to someone who has Windows XP, there may be some copyright issues that you'll have to deal with.

Why not simply download them from the net, loads easier..