I've seen this first hand last week. I was using Malwarebytes to celan up a friend's infected laptop. Malwarebytes found some stuff and asked for a reboot. After reboot, XP would blue screen within a couple of seconds after seeing the initial XP splash screen and then be stuck in a blue screen reboot loop. I toook the infected drive out of the laptop and connected it to my desktop computer and ran a full scan on it using Microsoft Security Essentials. MSE found a root-kit in ATAPI.SYS and cleaned it out. I put the disk back in the original computer and presto... it booted up into XP no problems. I'm just not sure if I installed any windows updates before the initial reboot after running malwarebytes. My web searching indicated that it's a combination of the rootkit not being able to run succesfully with the latest kernel or whatever, which causes the blue screen.