You seemingly miss some background.
A MBR is made of three main parts:
- Disk Signature (only on NT based systems) (DATA)
- Partition Table (DATA)
A "normal" MBR has additionally these two characteristics:
- it is 512 bytes long (i.e. it fits entirely in the first sector)
- its only scope is to chainload the first sector (PBR) of the Primary partition marked as actiive in the partition table.
Booting sequence for a "normal" MBR:
BIOS->MBR->PBR of Active Primary partition->loader or system file->OS
BIOS->MBR->PBR of Active Primary partition->NTLDR->BOOT.INI choices->NTDETECT.COM-> XP OS
BIOS->MBR->PBR of Active Primary partition->BOOTMGR->\boot\BCD choices->WINLOAD.EXE-> Vista OS
A Linux behaves differently.
The GRUB or GRUB2 can be installed BOTH to the MBR (but in this case it will take more than the first sector) or to the Volume/Partition (PBR).
IF the GRUB is installed to the MBR, it will look for it's own files, and then access it's configuration file (either menu.lst or grub.cfg) where the "choices" are.
IF the GRUB is installed to the PBR, the MBR may remain a "normal" one, still only chainloading the PBR of the active partition, which will then load GRUB.
We have to understand if your MAIN bootmanager is currently the Vista BOOTMGR or either GRUB or GRUB2, and how EXACTLY they are set.
TESTDISK (as well as other data recovery oriented tools) will only care about the partition table of a MBR (completely ignoring both the code and the disk signature).
Of course any install will NOT change the partition table (nor the Disk Signature, unless it is empty or having a collision with another disk).
The XP install will change the CODE of the MBR, replacing it with a "normal" MBR simply chainloading the PBR of the active primary partition.
It will also change the CODE in the PBR writing one that will load NTLDR.
It will "keep" the contents of BOOT.INI.
But if you installed those three Operating Systems in that order, the Vista install will have:
- replaced the MBR with some slighly different code (but that still chainloads the PBR of the active partition)
- replaced the PBR code with one loading BOOTMGR
- added to the \boot\BCD an entry corresponding to the one(s) you had before in BOOT.INI
The point is then which bootmanager Netrunner uses and HOW EXACTLY it was installed.
It could have been installed at least in two ways:
- to the MBR (and hidden sectors) and become the "main" bootmanager
- to the PBR (of another partition dedicated to the Linux OS) and added as an entry to the \boot\BCD