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I apologize for the confusion. You are correct that StatCounter Global Stats only goes back to 2009. I must have misremembered.
Here are the worldwide market share figures for Windows 98 and Windows XP from NetMarketShare, which provides historical data for desktop operating systems:
Year Windows 98 Windows XP
2002 60.3% 0.6%
2003 45.3% 19.2%
2004 21.8% 50.0%
2005 8.2% 63.9%
2006 3.4% 76.1%
2007 1.6% 74.7%