Windows NT was originally a microkernel architecture. NT4 moved a bunch of code back into the kernel space for performance reasons. Most notably: graphics and

Windows NT was originally a microkernel architecture. NT4 moved a bunch of code back into the kernel space for performance reasons. Most notably: graphics and printer drivers, which are not typically written the highest standard. Big iron vendors don't really have that problem, since they typically control their hardware as well. Microsoft had to rely on component vendors to provide driver software and couldn't plausibly test all permutations under all conditions (even though they test very, very many).