The only process I bother with any longer when XP steals the boot is repairing Windows 7 to boot so that EasyBCD can be installed to add the other OS's.
I've had no problem adding any other OS's including Win8 after this, although Win8 will give an improved Metro-style Boot manager if it is correctly installed last to configure the boot.
We've handled countless numbers of these without ever having to actually edit the BCD thanks to EasyBCD. However it has it's weaknesses. Using Easy to move the System boot files to another partition loses the F8 System Recovery Options
which consumers don't want to do, so it's best to stick with running Startup Repair - Run 3 Separate Times
after marking the target partition active.
And I won't mess at all with trying to add other OS's from XP using Easy because it increasingly fouls out and if pushed can corrupt it from being added once Windows 7 is restarted. Can you fix that?