Are you talking about the default font for say icons, desktop, etc.? Or the default font in a specific program?
If you are talking about the one for any specific program... you will need to go into that program and change its settings to have a new default (or old default) font.
If you are talking icons and desktop texts... Then you need to right click on the desktop background then then in the popup click "Personalize" or you can also go to "Control Panel" and then click "Personalization". Now click "Window Color". In here you are going to click "Advanced appearance settings".
Now click on the drop down under "Item" and click the item listed which you want to change the font of. Say "Menu" as an example. You will then see the size for the "Menu" itself to the right as well as the color. Below it you will now see the font it uses, the size of the font, the color of it as well as if it is bold and/or italic.
You can do this for many of the items listed in the drop down under "Items" and then when you are done click "Apply" and then click "OK". This window should close and take you back to the prior one. In that window click "Save Changes". Your fonts for these things should now be changed.