I’ve installed Windows Vista SP1 today. Unfortunately, the two first tries were unsuccessful, due to a message received after a few restarts (right after a "Configure Updates – stage 3 completed 100%" message and allowing log on after that) indicating an obscure error with this number: 0x800F0826.
After searching on the Web for the issue there was nothing that could be used.
I contacted Microsoft Support on their chat line (it’s free), and… surprise… they actually DID help me! The trick was to start 2 services in Control Panel Services, and set them to Automatic: Windows Event Collector and Windows Module Installer. After that SP1 has successfully installed.
I hope it helped other people like those from these forums or blog entries: