Microsoft Knowledge Base Article 903705 contains the following introduction:

After you install Windows XP Professional x64 Edition on a computer that is running a 32-bit edition of Windows XP, you may want to remove Windows XP Professional x64 Edition and continue to run the 32-bit edition of Windows XP. This article describes how to remove Microsoft Windows XP Professional x64 Edition from a computer that is also running a 32-bit Edition of Microsoft Windows XP.

Note This article applies only to a situation where you have installed Windows XP Professional x64 Edition on a separate partition from the 32-bit edition of Windows XP.