I've got a laptop that recently had an error loading the registry.
Windows XP could not start because the following file is missing or corrupt: \WINDOWS\SYSTEM32\CONFIG\SYSTEM
A repair of Windows fixed the problem. Once I got the OS to run, I downloaded a tool from the hard drive manufacturer and found that it passed all the tests on the drive.
A few days later, I booted the laptop and found the CD drive was not listed under "My Computer" I rebooted the laptop and the drive did show.
I'm thinking that the motherboard on this laptop is faulty, but I'm looking for other opinions.