The kernel source code (NTOS) was available for years through an academic program[1], it was easily findable on emule/edonkey/torrent. IIRC, the implementation of NTFS was not included. This leak is the full source code of Windows XP, both kernel and userland (+50 Gigabytes).
I also remember Windows source code leaking in the early 2000s. I even downloaded it myself back then but that was many computers ago.
And afaik it was not buildable. It was a large chunk of the source code that iirc had leaked from a 3rd party who was tasked with making some component, perhaps related to the image library.
I've heard for years that it was floating around, and saw some torrents that claimed to be the XP source, but I've never downloaded it myself to find out.