Microsoft released a new article explaining about why we cannot install Exchange Server 2003 on x64-based versions of Windows Server 2003.
It follows the More Information section of the article 924046 below:
The x64-based versions of Windows Server 2003 seamlessly support the running of 32-bit programs. However, on the x64-based versions of Windows, all the kernel mode components must be native 64-bit components. Therefore, all drivers must be 64-bit components.
User mode 32-bit programs are run under Windows on WOW64. WOW64 is the 32-bit emulator. However, Exchange Server 2003 will not even be installed correctly under WOW64. The primary reason is that Exchange Setup installs the 32-bit driver for the Installable File System (IFS) that is incompatible with the 64-bit kernel.
This article can be found at https://mvp.support.microsoft.com/default.aspx?scid=kb;en-us;924046