How do I change system fonts in Windows 7

This takes just a few steps that make it pretty clear why no one can find this.
Open Control Panel.
Now, in the search box for the Control Panel, enter "window color":

In the search results, click on the link Change window colors and metrics.
The result:

To those of you who are used to making these changes in earlier versions of Windows, such as Windows XP, this is a very familiar dialog. Other than the steps for locating it, not much has changed.

Window Color and Appearance and Aero

As you can see by the text in the dialog itself, one thing that has changed is the addition of the Aero theme to Windows 7.
Aero used advanced techniques to display many of the items that would normally be controlled by the settings in this dialog. As a result, the Window Color and Appearance dialog does not apply when using Aero; you'll need to be using one of the more traditional styles, as indicated.

Window Color and Appearance ... and more

One common question that I've received from people moving to Windows 7 from Windows XP is how should they control the spacing of icons on their desktop and elsewhere.
That also is covered by the Window Color and Appearance dialog:

In fact, there are several setting that are adjustable here:

If you want to experiment here, that's fine. Just make sure to write down or otherwise save the original settings so that you can always revert back to them.