Windows Division revenue is largely correlated to the PC market worldwide, as approximately 75% of total Windows Division revenue comes from Windows operating system software purchased by original equipment manufacturers ("OEMs") which they pre-install on equipment they sell. The remaining approximately 25% of Windows Division revenue is generated by commercial and retail sales of Windows and PC hardware products and online advertising from Windows Live.
Windows Division revenue reflected relative performance in PC market segments. We estimate that sales of PCs to businesses grew approximately 2% and sales of PCs to consumers declined approximately 6%. Excluding a decline in sales of netbooks, we estimate that sales of PCs to consumers grew approximately 2%. Taken together, the total PC market decreased an estimated 2% to 4%, including the impact of supply chain issues from the Thailand floods. Windows Division revenue was also negatively impacted by a reduction in inventory levels within our distribution channels as well as by the effect of higher growth in emerging markets, where average selling prices are lower, relative to developed markets, and by lower recognition of previously deferred Windows XP revenue.
Windows Division operating income decreased primarily because of lower revenue and higher research and development expenses. Research and development expenses increased due mainly to an increase in headcount-related costs and product development costs associated with the next version of the Windows operating system.