Microsoft online product activation was first introduced in Office 97, a few years before WinXP.
Win2K was the first one for which I've heard about it personally, probably because that's when it started to show up en masse in developed markets.
https://en.wikipedia.org/wiki/Microsoft_Product_Activation
Let's rephrase @endgame's comment as "Windows XP and Office XP were the first versions of their respective series where product activation was mandatory on all retail editions." Point still stands I guess.