1.Right-Click desktop->Personalize
2. Choose "Window Color" from along the bottom
3. Select "Advanced Appearance Settings".
You can change font sizes within this dialog. For example, if you wanted larger icon text, but wanted to leave the icons the same size, you would choose "Icon" from the "Item" combobox; then you can change the font and size. You can do this for quite a number of elements. You can also make caption buttons larger.
Just noticed JJ3000 already posted this.
Oh well. Also, it blabs about it not working for Aero, but that's a outright lie; most of the settings work for Aero (with only a few exceptions, such as setting the active/inactive title bar colour, for example)