A friend of my wife has a PC which is giving endless problems, she wants me to do a full restore which will take it back to a pre Servicepack 1 version of XP. Question - will installing SP2 from CD alone bring it right up to date without installing SP1 first?
Windows Update should be intelligent enough to work out what you need, when you install SP2. It will probably be a bigger download if you've rolled back some of the older updates, but you shouldn't have to install them explicitly.
Thanks sjbell, that was the definitive answer I was looking for. Quote from the Microsoft Knowledge Base
This article lists the problems that are fixed in Microsoft Windows XP Service Pack 2 (SP2). Service packs are cumulative. This means that the problems that are fixed in a service pack are also fixed in later service packs. For example, Windows XP SP2 contains all the fixes that are included in Windows XP Service Pack 1 (SP1). You do not have to install an earlier service pack before you install Windows XP SP2.
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