Oh ok.

Resource Hacker is a free tool to edit Windows resources from binary files.
1. Download it here:
http://www.angusj.com/resourcehacker/.
2. Start Resource Hacker as administrator. (Opening it as admin is important because the skin files are located in C:\Program Files\Classic Shell\Skins\ and are protected by UAC virtualization.
3. Then open the skin in Resource Hacker whose font you want to modify by going to File -> Open -> C:\Program Files\Classic Shell\Skins\<name of skin.skin>. (You may need to change "Files of type: All files" in Resource Hacker's Open dialog.
4. Once the skin file is opened in Resource Hacker, locate the resource type called SKIN. Expand it by clicking + sign. Expand the resource named 1 under it. Click 1033.
5. Now you can press Ctrl+F or do "Find Text" from View menu. Type: font in the Find Tex dialog. Whatever values of fonts you see e.g. Caption_font, User_font etc, change them as desired. e.g. if Caption_font="Segoe UI",normal,18, you can make it Caption_font="Arial",bold,16.
6. Finally, click Compile Script.
7. Exit Classic Start Menu by right clicking the Start button and clicking Exit (exiting Classic Start Menu is recommended because if the skin is in use, it may cause an access violation and cause the skin to get corrupted while saving).
8. Then in Resource Hacker's File menu, click Save. Resource Hacker save the changes to the skin and will also automatically create a backup copy of the original skin should you wish to revert to it.
9. Start Classic Start Menu again from C:\Program Files\Classic Shell\ClassicStartMenu.exe