If you are a student, you school or university is probably member of the MSDN Academic Alliance, and you can download some Microsoft software on the Microsoft website for free, and maybe XP x64 is part of it.
When you install an earlier version of Windows after having installed Vista, you won't be able to boot Vista anymore.
You have to use the installation CD of Vista and enter in the prompt the command :
bootsect /NT60 ALL
in order to install all versions of Windows in the Vista Boot Manager.
Moreover, I don't know exactly why, but I was not able to boot directly XP x64 on its logical parition with Grub (error 12). I have to boot it through the Vista Boot Manager.
You can install x86 version, but if you want install 64 bits version, you can download it here : http://www.mozilla-x86-64.com/download.html.
Graphic card and sound card are not recognized by Windows x64, and Windows Update can't find it neither.
I had to download nvidia forceware-96.89-beta-wxp64.exe for the chipset (sound card), and to install the driver for GeForce4 440 MX in the Windows list of devices for the graphic card, which seems compatible with GeForce4 440 Go (Go is the mobile version, MX the desktop version).