How and when Vista decides to configure its own bootmanager.
The rules that Vista uses for deciding to set up dualboot are in most regards the same as earlier WinNT operating systems and awareness of the how and the why should help you avoid surprises. If you select a partition for the install that is not the Active Primary Partition on the boot hard drive then the Vista setup will look to see if there is another partition marked as active on the boot drive. If one is found that is FAT16/32 or NTFS then crucial Vista boot files (bootmgr and BCD) will be placed on this active partition and the OS will go to the chosen partition. During setup the active partition will be examined for a previous install of WindowsNT and if one is detected then the Vista boot files will be configured to include it in its boot menu. If you already have a previous Windows dual or multiboot running from this primary partition then Vista will retain the old setup and you will then have two boot menus at start up. The first Vista boot menu will have two entries where you can choose Vista or your previous OSes. When you select the previous OSes you will get your original boot menu.
The Vista install can only place its boot files in a suitable primary partition on the drive designated as the first hard drive in the bios boot order. It’s for this reason that you cannot do a fully independent install of Vista to a logical partition or another hard drive. If the boot drive does not already have an active partition then the primary partition or unallocated space chosen for the install will be made the active partition, (Windows Seven behaves differently with unallocated space - see Installing Windows7). If another active partition is already present and receives the Vista boot files, then it will have its Partition Boot Record (PBR) changed to load the Vista files instead of whatever it was previously set to load. Only the active partition on the boot drive will be modified and no other previous Windows OS on any partition or drive will be touched or added to the Vista boot menu, but another Vista OS might be even if its partition is hidden. There is a full guide to the Vista multiboot method of operation on this page.
Hard Drive Numbering
There are three main changes made to the original set up. The Vista boot files are placed in the root of the active primary partition on the boot hard drive. The PBR of this partition is altered to load bootmgr instead of the ntldr of the previous OS. The MBR on the boot hard drive is replaced with the Vista one. The only action essential to remove Vista and restore the original boot sequence is to reinstate the previous PBR of the active partition. With XP/2K/2K3 this is running one simple command from the Recovery Console – fixboot. Once you have done this and the machine is booting as before with no Vista boot menu you can safely delete the Vista OS and its boot files.
The five Vista files in the root of the original OS will cause no problems if left in place, but they can be deleted if desired. They are:- Boot.BAK - BOOTSEC.BAK - $RECYCLE.BIN - bootmgr and Boot (a folder). Don’t of course confuse these with the original boot.ini file and accidentally delete this as well. Vista will have added a couple of lines of text to the start of the boot.ini just to inform users that editing it will not affect how bootmgr operates. These can be deleted if you know what you are doing. The Vista MBR will happily boot previous Windows versions with no problems and does not need to be replaced for Windows sake. However, many current disk manipulation apps for partitioning, imaging, backup etc may not yet be fully compatible with this MBR, so it may be advisable to restore the original. Again one command from the Recovery Console will do the job – fixmbr.
When installing Vista to the boot hard drive all that is required to prevent the configuration of the Windows bootmanager is to make the target partition for the install the Active partition on the hard drive. Any other install of Windows will then be ignored and the Vista boot files will go to the Vista partition. Hiding the other partitions is not necessary and would make no difference anyway if you had got your configuration wrong, as the Vista DVD seems to completely ignore the usual hide flag. The Vista setup only looks at the Active partition and if there is no part of another Windows install to be found there then it will look no further. Even another Vista OS or any current Windows dual or multiboot arrangement on the other partitions will have no effect. (Note: When installing any previous WinNT in this manner it is advisable to hide other primary partitions).
So before you start an install, create and format the desired partition for Vista and make it the active one on the disk, then boot the computer from the Vista DVD and select your new partition as the destination for the install. You can only install this way to a primary partition as logical partitions cannot natively be set as active. You cannot install completely to a second or higher hard drive as the Vista boot files must be placed on the boot drive. The only way to install independently to another drive is to make that drive the boot hard drive during the install, so either physically move its position in the computer or change the bios to make it the boot drive. (You cannot do an independent install of Windows if you run setup from inside another Windows OS.)
There have been reports that some partitioning tools don't format in the correct version of NTFS that Vista requires, which has resulted in some problems. If you don't know about your own tool, then either format before hand with the XP or 2K3 Disk Management or Diskpart tools, or when you get to the point in the Vista setup where it asks "Where do you want to install Windows?" click the option in the lower right of the screen "Drive options (advanced)" and then highlight the relevant partition and choose 'Format' and this will reformat the partition in the correct 3.1 NTFS version without affecting the Active status you have given it. You can then click 'Next' to proceed with the install.
I've also been experimenting with installing to unallocated free space and to unformatted partitions and so far this has also been successful, (Windows7 handles free space differently - see installing Win7). It seems if there is no active partition at all on the drive then the Vista setup will make the destination for the install the active partition and still ignore everything else. I have not tested this extensively enough to guarantee it is applicable in all situations. (With XP etc. you must create the partition before hand and set it active). There is an issue with Vista created partitions that will 'mis-align' them with other partitions, which is not a good situation on multiboot machines, so for now I would recommend that you do not allow Vista to create your partitions. (Update:- do not follow this avice for Advanced Format hard drives.)
Installing Windows without the bootmanager presents the problem of how do you then select which operating system to boot. For boot drive primary partitions this can be done by changing the active partition around, either with Window's own Disk Management utility or any app that can do it from Windows or boot disk. For a second or higher hard drive the booting choice can be selected by changing the bios boot order as and when required, either by entering the bios setup or from a bios boot menu accessed by pressing a function key during early POST. These methods are not ideal and a better solution would be a third-party bootmanager. Vista Tested Bootmanagers. If you don’t want to install a boot manager on the hard drive then Boot-US on floppy or GAG from floppy or CD will do the job. Another option would be to make a Vista Boot Floppy which works similar to how the XP boot floppy did. If you don’t have a floppy drive in your computer then perhaps a virtual floppy on a boot CD like the Ultimate Boot CD may work, but I have not tried this.
There has been an important change to the way Vista boots and it is relevant to the type of bootmanager that can boot Vista from a second or higher hard drive. With an independent install of earlier WinNT on a second or higher drive you need a bootmanager that can 'Drive-Swap', which basically emulates the bios change to make Windows believe it is indeed on the boot hard drive. This is necessary because the ntloader will only work from the boot drive. With Vista however drive swapping is not required because the new bootloader winload.exe is always inside its own install and so has to be able to work from whichever hard drive it finds itself on. This means that many currently available bootmanagers that cannot drive-swap will still be able to boot Vista from any connected hard drive. This would have been an advantage for multibooters but unfortunately there is another new and unique Vista issue that means drive swapping is still desirable. See BCD is Always Open.