Check this link: https://web.archive.org/web/20130208113614/http://blogs.msdn.com/b/vcblog/archive/2012/10/08/10357555.aspx
original link (dead): http://blogs.msdn.com/b/vcblog/archive/2012/10/08/10357555.aspx
It explains how to compile in command line using cl.exe and choose XP toolset.
When compiling boost, before starting the build itself using b2.exe, insert all required parameters in order to choose 120_xp toolset.