Windows XP Embedded is still supported and still gets security patches. I'm also pretty sure you can get the source to it too, if you pay Microsoft and sign an NDA.
Windows XP; the security researchers favorite distro.
There are few things better than showing up to a security review gig and have them running XP. Makes my job super easy and clients love when I rain down bugs. It's even better because we can actually write exploits in a short time frame. Love XP.
