I still keep my 12 year old legal copy of Windows XP available to be run as a VM via VirtualBox for occasions when I need to try out some Windows/DOS stuff - works pretty well :)
" Legal notice: If you have seen proprietary Microsoft Windows source code (including but not limited to the leaked Windows NT 3.5, NT 4, 2000 source code and the Windows Research Kernel), your contribution won't be accepted because of potential copyright violation. "
Edit: Might of course also just be covering their backs legally... but who am I to say.