The New York Times is running an interesting article on why they think Windows is so slow. Here's an excerpt:

Windows runs on 330 million personal computers worldwide. Three hundred PC manufacturers around the world install Windows on their machines; thousands of devices like printers, scanners and music players plug into Windows computers; and tens of thousands of third-party software applications run on Windows. And a crucial reason Microsoft holds more than 90 percent of the PC operating system market is that the company strains to make sure software and hardware that ran on previous versions of Windows will also work on the new one - compatibility, in computing terms.

As a result, each new version of Windows carries the baggage of its past. As Windows has grown, the technical challenge has become increasingly daunting. Several thousand engineers have labored to build and test Windows Vista, a sprawling, complex software construction project with 50 million lines of code, or more than 40 percent larger than Windows XP.

"Windows is now so big and onerous because of the size of its code base, the size of its ecosystem and its insistence on compatibility with the legacy hardware and software, that it just slows everything down," observed David B. Yoffie, a professor at the Harvard Business School. "That's why a company like Apple has such an easier time of innovation."