I will assume you are talking about how to change your font on your desktop. Take a look at this to see how to Customize the Windows Vista Desktop
Customizing your computer is a way to show your personality and interests and also means you can keep settings the way you want them. By changing your desktop background image, font and sidebar options, you can make your Windows Vista desktop feel like your own.
1. Right-click the Windows Vista background. An options box pops up.
2. Click on "Personalize," and then on "Desktop Background," which takes you to another options box.
3. Choose the image type you want from the drop down location menu or browse for the specific file folder in which you store your images and photos. Windows Vista loads some background choices onto your computer automatically so you are sure to find something that interests you, or you can choose your own favorite photo.
4. Select the background image you want for your desktop.
5. Decide how you want the image to be positioned on the desktop--for example, tiled, centered or stretched--and click the appropriate button. Vista allows you to see what the image looks like in each option, so comparison is easy.
6. Click "OK" and watch your image change. Repeat the steps to change the desktop image again.
Change the Desktop Font Size
1. Click the Start button and then "Control Panel."
2. Type "Adjust font size" into the search box. This takes you directly to the "Personalization: Adjust Font Size (DPI)" option box.
3. Click "Custom DPI" at the bottom corner of the box.
4. Adjust the percentage to normal size until you see the size you prefer and click "OK." Go through the steps to change the size again or return it to normal.
Change the Sidebar Options
1. Open Windows Sidebar by clicking the Start button, selecting "All Programs," "Accessories," and "Windows Sidebar" or locate the Sidebar icon in the taskbar, right-click and select "Open."
2. Right-click the Sidebar or a detached gadget and click "Always on Top" to keep the Sidebar or specific gadget visible even when you open another window or program.
3. Add gadgets by right-clicking the Sidebar and clicking "Add Gadgets."
4. Place a gadget on the desktop by right-clicking the gadget and selecting "Detach from Sidebar." To move it back, right-click and select "Attach to Sidebar."