I have always wondered why Windows updates are so incredibly slow.

Windows XP was the worst — after installing, you had to update, reboot, update, reboot, update, reboot, … Even on a modern computer with broadband, it takes an entire day to install and fully update.

Windows 7 is much better, but it's still like treacle in comparison to Ubuntu, and that's even ignoring the fact that you don't have a package manager for the various non-Microsoft programs.

I am just curious. Why are Windows updates so time-consuming? Surely updates are just downloaded files and, for Windows, updates to the Registry? Or does Windows do all kinds of weird and wonderful stuff when updating?