I haven't heard of box files being offered since Digital River caused an uproar by issuing them instead of ISO's at Windows 7 release. MS only offers ISO's that I've seen.
May I ask which burning tool can make an ISO out of box files? ImgBurn can only burn the ISO (flawlessly) or extract an ISO from disk as far as I know.
We used this tool previously to compile files into ISO: Boot to CD/DVD