from XP are doing so due to the cost of important legacy software. A few I know that were concerned about having move onto what they call Microsoft Orifice, have been successfully moved up to Win 7 with Libre Office - the move to Win 7 was due to some hardware driver issues between XP and their new PCs.
There are a lot of organisations with critical legacy software originally designed for Win 2000 and Win XP that do NOT have newer versions as much of it was special design stuff. All that is very costly to redo.
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