Read this
http://download.microsoft.com/download/9/c/5/9c5b2167-8017-4bae-9fde-d599bac8184a/OEMBoot_Vista.doc
If you buy a retail copy of Vista/7/8 it comes with the default MS bootmgr module, which is the only one available to EasyBCD.
If you remove the customized version which your OEM probably provided in its place, EasyBCD cannot recreate whatever additional proprietary features the OEM included.
It's probably still available somewhere in your recovery partition and no doubt, with sufficient inside knowledge and appropriate tools you could locate and restore it.
EasyBCD, 99% of the time, is merely modifying the contents of your BCD in accordance with your instructions. It's not a boot manager, just a tool to help you manage the one on your PC. It does however enable you to recreate the
standard bootmgr module should you accidentally (or deliberately) destroy the existing "live" copy.
In your situation, bearing in mind that the MS development team have also done a lot of
under the hood tinkering with the W8 boot which complicates things even more, I'm just saying I'd be very wary about overwriting it all with an XP install and then trying to get it all back again.
With W7 on an MBR configured disk I'd advise you to install XP into its own partition,
having set it active first, to prevent it from touching the existing boot ( MS setup always installs the boot files to the "active" partition -i.e. normally a pre-existing
older OS, which is how it contrives to auto dual-boot systems), then you could set the original system active afterwards and add the new XP as a BCD option in the untouched original boot partition.
Since GPT disks don't have an "active" flag you can manipulate in that way, the option is not open to you on a contemporary UEFI W8 PC to do the same.
I'm just warning you, things have got a lot more complicated since that guide was written, before UEFI, W8, GPT or "fast boot" had even been thought of, and nowadays you'll need to carefully weigh the possible consequences against just how important it is to have a native XP on your PC.