I’ve always been curious about this. It seems that there must be more than simple file copy operations happening. I just reinstalled XP on my netbook (Atom proc, 1GBmem) and installing all the various updates took forever. This morning, I installed .net 4 and then it found 8 different updates to that. It has been installing those 8 updates for well over 15min now. If it were just replacing files or making registry tweaks, surely it wouldn’t take this long? Is it compiling stuff? What could be taking this long?
The issue is that there are a large number of interdependencies among Windows components and when one of them is updated, all of the things it depends on and all the things that depend on it have to have their assemblies rebuilt. Also, everything is checkpointed so that a failed update can be rolled back. That adds quite a bit of time as well.
This is in addition to the actual process of making sure the update applies to the system, unregistering the files that need to be patched, patching each individual file, and then re-registering the files.