If the source code for Windows XP were open source, independent security researchers and developers would be able to proactively analyze it to find and fix flaws rather than waiting for discovered vulnerabilities to be patched by Microsoft. If Microsoft were to sponsor or coordinate that effort—or even just cooperate—the open source community could be given early access to vulnerability data so that patches could be developed for XP in parallel with the supported operating systems, and released simultaneously along with the monthly Patch Tuesday security bulletins.
Of course, there are some very good reasons not to make Windows XP open source as well.