In tip 3920, I described performing an in-place upgrade of Windows 2000.

Microsoft Knowledge Base Article 315341 contains the following INTRODUCTION:

This article describes how to perform an in-place upgrade, or reinstallation, of Microsoft Windows XP. This is also named a repair installation. It reinstalls Windows XP to the same folder. You may want to perform an in-place upgrade if your installation of Windows XP must be repaired. Such a repair installation may be required if one of the following conditions is true:

You cannot start Windows XP in Safe Mode.

For additional information about how to start your Windows XP-based computer in Safe Mode, click the following article number to view the article in the Microsoft Knowledge Base:

315222 A Description of the Safe Mode Boot Options in Windows XP

The problem is caused by a recently installed Microsoft software update, and you cannot solve the problem in any other way.
There is a registry problem that cannot be solved by using other tools such as System Restore.
You must apply default (file and registry) permissions to your Windows XP installation.
You must register Component Object Model (COM) components and Windows File Protection (WFP) files.
You must use the Windows Setup program to enumerate Plug and Play devices again, including the hardware abstraction layer (HAL).
To reinstall Windows XP, use the appropriate method in the "More information" section.