Microsoft's official support for Windows XP ended in April, 2014. Now, almost two years later, Blender developers are also forced to drop XP support. Developer Martijn Berger wrote on the maillist:
We implicitly dropped Windows XP when switching to python 3.5.
Python has the so called PEP system and PEP 11 "Removing support for little used platforms" defines the support for windows XP to be over at the same time Microsoft stopped supporting it. Since python 3.5 released after XP died it does not support XP.
While it might be possible to backport python 3.5 to windows XP i think
that we should not do this.
So blender 2.77 will not run on XP. The 32 bit release requires Vista or
newer and the 64 bit release Windows 7 SP1 or newer.
— Ton Roosendaal (@tonroosendaal) February 22, 2016
Windows XP is still being used in the Blender community, but its contribution is tiny:
— Ton Roosendaal (@tonroosendaal) January 5, 2016