The New York Times hit the nail on the head:
When designing Windows Vista, Microsoft put a lot of effort into taking advantage of sophisticated computer hardware, with fancy graphics on the surface and lots of new processing tricks inside. Microsoft says this positioned the operating system for the future, despite the complaining of many users who grumble that Vista has bogged down their computers with unneeded frills and incompatibilities.
While there is some more high-powered glitz coming in Windows 7, the real challenge for Microsoft isn’t the latest multi-core superchip, but making the operating system work well, and affordably, on stripped-down PCs, netbooks and other small devices.