If you want change 2000, XP x86's version and build number, use imagecfg! for Longhorn, modify ntoskrnl or ntkr**** via hex editor. (follow this guide!)
But in case of XP x64, imagecfg doesn't work and I can't find where major and minor are in ntoskrnl..
Does anyone know where they are? I was able to find build number in ntoskrnl, so version is 5.2.2600 now