Owners of Vista Ultimate and Vista Enterprise that also run a non-Microsoft operating system on the same PC may not be able to install Vista Service Pack 1 without changing the boot process of their machines.

The Service Pack, which was released last week, can’t be installed before up to three other updates are put in place.

One of them, KB935509, requires access to both the master boot record and system bootloader on which Vista is installed. In a multiple operating system environment, one or neither may be available.

A thread on Microsoft’s TechNet forums reveals that the situation is due to the BitLocker feature which is present only in Vista Enterprise and Vista Ultimate.

Michael Kleef, Microsoft technology advisor, told Builder AU: “[When using BitLocker] Vista needs to be in control of the entire boot sequence to maintain a chain of trust — from the MBR (master boot record) through to the bootloader, access to the TPM (trusted platform module) and finally the system boot. If there’s anything in the way of that, that violates that chain of trust: problems are likely to occur which could result in boot failure should anything unexpected occur […]

“When the servicing update sees that the MBR is not using our bootloader it fails the update so as to prevent a problem with either overwriting the current MBR configuration or potentially a BitLocker-based boot failure.”

The KB935509 update will fail regardless of whether the user currently utilises BitLocker or not and attempts to remove it will not avert the problem, as the update still requires access to the MBR, under the assumption that the user may choose to use BitLocker in the future.

The problem will occur when attempting to install either the KB935509 update or the Service Pack itself.

To allow the update to be successful, users will have to replace the other bootloader, such as grub or LILO, with the Vista bootloader — details of such a procedure can be found here.

Users which keep Vista on a separate hard drive to their other operating systems can avert the problem by setting the hard drive to boot to the Vista drive in their BIOS.