Why would Windows XP not start after an improper computer shutdown due to a power failure?
Probably due to a hardware failure or driver corruption due to
the power failure. First try to boot in safe mode (press F8 during
boot process, chose "safe mode" from the menu). If that doesn't
work, open the case and remove all adapter cards except video.
Unplug all drives except main hard drive. If Windows will then
start, shut down and add one component at a time until the problem
returns. That component will probably be the culprit. If removing
the cover scares you, do not do it, call for help.