Hi there
I think if you want Windows XP 64 your best bet is this site :
Software Library : Free Software : Free Download, Borrow and Streaming : Internet Archive
There are some older OS'es on the site with the key numbers so you could be lucky. Just search through the archive.
The main problem with XP 64 is that there are loads of drivers missing and the 32 bit ones won't always work.
As a VM "paravirtualisation" of video etc can solve a lot of the driver problems if you use VBOX / VMWare vmplayer --both free and sometimes if you have to use "native drivers" the drivers from Windows 2003 server will possibly work e.g for some printing.
If you want to play around / experiment with XP IMO it's best to simply use the 32 bit version. It runs just fine as a VM and consumes almost zero resources on any decent hardware,
Cheers
jimbo