Microsoft Knowledge Base Article 307545 contains the following summary:

This article describes how to recover a Windows XP system that does not start because of corruption in the registry. This procedure does not guarantee full recovery of the system to a previous state; however, you should be able to recover data when you use this procedure.

Warning Do not use the procedure that is described in this article if your computer has an OEM-installed operating system. The system hive on OEM installations creates passwords and user accounts that did not exist previously. If you use the procedure that is described in this article, you may not be able to log back into the recovery console to restore the original registry hives.

You can repair a corrupted registry in Windows XP. Corrupted registry files can cause a variety of different error messages. See the Microsoft Knowledge Base for articles about error messages that are related to registry issues.

This article assumes that typical recovery methods have failed and access to the system is not available except by using Recovery Console. If an Automatic System Recovery (ASR) backup exists, it is the preferred method for recovery. Microsoft recommends that you use the ASR backup before you try the procedure described in this article.

Note Make sure to replace all five of the registry hives. If you only replace a single hive or two, this can cause potential issues because software and hardware may have settings in multiple locations in the registry.

If you experience a repetitive occurrence of registry corruption related issues, Microsoft recommends that you obtain and install the hotfix in the following article:

318159 Damaged Registry repair and recovery in Windows XP

This update is also included in Windows XP Service Pack 1.