The MBR has only 512 bytes.
Every PC operating system can restore its own MBR, including Windows and Linux of course.
You can use a Dos floppy to restore the MBR for the entire MS family OS, although officially the advice is to use the installation CD.
You can boot both your Windows Xp and Xp64, say they are in hda1 and hda2 partitions respectively, by adding (or ensuring) these lines in the file /boot/grub/menu.lst of the Linux if it uses Grub as the boot loader
title XP in hda1
title xp64 in hda2
My guess is you should be using Xp to dual boot Xp64 and that should remain operative.
If your Linux uses Lilo lest us know.
You need to edit the /boot/grub/menu.lst with an editor while inside Linux and log in as root, say using the editor "vi". The command is just
If you have installed the OS successfully then you can boot all of them using a boot loader from any of the OS. However Linux is much easier to work with.
The 5 piece partitions are OK as you must have some of them in logical partitions.