Yesterday I ran into a tricky problem whilst reinstalling Windows Vista onto a larger hard disk for my gaming system. The service pack spent about forty minutes installing and then failed with the message "Service Pack did not install. Reverting changes." The error code in the Windows Update log was fairly generic and not indicative of a specific problem. Repeated attempts to install all failed with this error.
The problem turned out to be quite odd. I stumbled upon a solution during a somewhat desperate Google crawl. SP1 will apparently fail to install if the Vista installation is not set as the primary hard disk in the BIOS. I have GRUB on another drive to chainload Vista, for dual-booting Linux, which works fine in normal use. However, in order to install SP1 I had to temporarily set the Vista drive as the primary hard disk and then put it back after installation.
I guess it's a little much to expect Microsoft to test their software's compatibility with bootloaders associated with competing operating systems.