MinWin is not so much a product as it is a continual effort starting with Vista to untangle dependencies and move unnecessary cruft out. It's still the NT kernel, only progressively cleaner with each additional release of Windows.
http://www.codinghorror.com/blog/2006/07/creating-smaller-vi...
Really cool stuff.
edit: s/XP/NT/
That was basically the bare minimum needed to run the OS on my laptop with wifi and sound.
Put something similar on my EeePC 701 when I got it, having a 6/7 second boot time on Windows was quite nice;