Try going to www.support.microsoft.com and click on Knowledge Base Article ID Number Search. Enter this article ID: 275704.
It does say that is applies to Windows 2000, but Win2K and XP share a lot of code. The suggested workaround in the article seems drastic to me. You may want to backup your system, or at least all of your important files and documents, then try running setup by booting from your XP CD. Setup will start and give you the option to repair the existing installation of Windows.
Hope any of this helps. Good luck!