It should be possible, but i cannot find a way to do so.

No 64bit PE cannot be executed frm a 32bit OS ((Kernel))

so u must boot frm CD

Frequently asked questions about Windows XP Professional x64 Edition

Q.  Can I upgrade from 32-bit Windows XP Professional to Windows XP Professional x64 Edition?

A.

No, you can't upgrade. You can use the Technology Advancement Program until the end of July, 2005, to exchange your copy of Windows XP Professional for the x64 Edition of Windows XP Professional, but doing so replaces your existing license with the new license. The actual installation of Windows XP Professional x64 Edition is a fresh install, not an upgrade. All your programs will have to be reinstalled, and if you have data on the same drive as your operating system, it will be wiped out. Make a thorough backup of your important files before doing the installation.

to lostincyberspace2007:

i don't wanna upgrade my system, i gonna have a new installtion, but i don't have a cdrom.

So there are two ways:

A. Install it from within WinXP or command prompt.

It should be possible, because the first step of windows setup just copies files and modifies boot.ini.

B. Build a Preinstall environment of windows x64.

Unfortunately, i've found no PE Builder supports windows x64.

To answer your question. "no." If u want to install Windows XP Professional x64, you can't install it over (upgrade) Windows XP Professional x86 version... it is not allowed... so to solve this problem u have to do a clean install of Windows...