If you use a web-based email service you're restricted to the font (or fonts) made available by that service. To use more fonts you'll need to use a proper email client, such as Windows Mail.
Click 'Outlook/Outlook Express Users' here (which should still apply to people using Windows Mail in Vista).
If you used Comic Sans with the old(classic) version of BT Yahoo Mail you can always revert to it. I only know how to revert using XP, it may be different in Vista.