I don't know about Vista, but on XP and earlier you can set the desktop font sizes by right clicking on the desktop, selecting "Appearance" tab and adjusting the contents of the "Font Size" box.

Also, under "settings" tab, click the "Advanced" button and you can change the DPI setting. This (if available) might be a better choice if every font is displaying too small (in all applications, not just the desktop).

Anahata