The utilities that come with Windows can assist you in tracking down problems. The Eventviewer utility is a good place to start looking as it records system, security and application events. This will tell you what device or service did not start if you have a message box popping up after logging on saying a device/service failed to start. Using the logging option or debugging option can also be used to identify where the problem in booting exists.
Another extremely useful source of information is the Windows Help and Support system.