Just boot W7, add an entry for XP in the BCD (let EasyBCD auto configure) and click "save entry".
That's it. Dual booting with W7 as the default.
EasyBCD copies everything necessary from XP into the W7 boot sector. (XP will be unaltered and continue to be stand-alone bootable via the BIOS).
You can use EasyBCD "edit boot menu" to make cosmetic changes to the displayed names, the display order, the default system or the timeout duration, but nothing fundamental changes on either system, only BCD entries are manipulated to your requirements.
Each time XP is booted and able to see W7, it will destroy the W7 backups and restore points. (they both use the same folder name "System Volume Information" but in entirely different incompatible formats). On rebooting W7, it will be forced to repair its own folder(s) and in so doing will break XP's chain and hence destroy XP's points in a never-ending tit-for-tat.
You must prevent XP from being able to see Vista/7/8 by the registry hack.
Set each OS to turn on system restore on the OS partition (and any partition containing apps installed, if that's not the same place) and off for all other partitions.
It's fine for Vista/7/8 to see XP. They're written to be backward compatible and will leave XP's SR points alone, but XP sees the later folders as different (hence corrupted) and will "fix" them for you. Just turning off SR on the W7 partition isn't sufficient. You must prevent XP even knowing that W7 is there.
After doing the above, set a restore point on W7, give it a name you'll recognize, boot XP, boot W7 again and check that your restore point is still there.