Are you talking about e-mail? Go to Outlook, then tools, then options
On the Mail format tab in the middle of the box, you can set your default fonts (and colors and sizes) for reaging and creating e-mail. You can also choose stationery if you want. Just realize that some people's spam filters will catch your pretty background and flag it as potentially hazardous.
Here is how to change windows fonts in Change Windows fonts in Vista Home, lovingly known to its users as ^&&*((@^^$^ piece of @^$^^%
You can change the font in any part of Windows (for example, menus). Changing a Windows font does not change your program fonts.
Click to open Appearance Settings.
Click Advanced.
In the Item list, click the part of Windows where you want to change the font. For example, if you want to change the menu font, click Menu in the list.
In the Font list, click the font you want to use.
In the Size list, click the font size that you want.
In the Color list, click the font color you want.
Repeat steps 3 through 6 for each part of Windows where you want to change the font, its size, and its color, and then click OK.