Just thought I'd post a useful link concerning differences in the XP and Vista MBRs, and whether the XP version can load a Vista boot sector (which searches for bootmgr) or not:
Summary of results of running Win98 SE's fdisk /mbr on a Vista computer.
(note that the computer multi-boots between two Vista installs, one with SP1
and the other w/o SP1, and several installs of XP)
1) Did the disk signature change?
Yes, and Vista and XP automatically changed the partition signatures to
reflect the change (per a registry review).
2) Did vista load and run in the normal manner with the Win98 SE MBR and the
new disk signature?
For the Vista install w/o SP1: yes, it had zero problems.
For the Vista install with SP1: no. It would only boot into a temporary
profile, using either of two admin user accts. The drive enumeration system
was changed so that the Vista SP1 vol instead of being C: (the pre-test
drive letter), the vol now boots as I:. Note that both Vista vols were
installed via a DVD boot using the same TechNet DVD, so, the drive
enumeration system was not changed for the Vista vol that did not have SP1.
3) Manually changing the disk signature in sector 0 (the MBR) back to the
pre-test signature did not solve the problem with the Vista SP1 vol.
For non-Vista booting, drive C: is XP Home, meaning that Home was installed
before the other op system installs.
The computer has two hard disks, one has a single primary partition and an
extended partition, the other only has an extended partition (no primary).