The source of the Windows Guest Additions is located here https://www.virtualbox.org/browser/trunk/src/VBox/Additions/WINNT
This directory contains code from Oracle, which is licensed under GPLv2. e.g. https://www.virtualbox.org/browser/trunk/src/VBox/Additions/WINNT/Mouse/NT5/VBoxMFDriver.cpp
However, there are many files in here that are attributed to Microsoft, e.g. https://www.virtualbox.org/browser/trunk/src/VBox/Additions/WINNT/i8042prt/kbddep.c The Microsoft files don't have an attached license, so they might not be free (IANAL of course).
