Microsoft confirmed that their upcoming Internet Explorer 9 will not run on Windows XP. Somewhat ironically, this makes Microsoft the first of the few big browser-makers to drop support for Windows XP, which is the world's most popular operating system, even though it is about 9 years old.
The move, according to Microsoft, is necessary because "Internet Explorer 9 requires the modern graphics and security underpinnings that have come since 2001," according to a company spokesperson. This comment is backed up by the fact that Windows XP lacks certain DirectX APIs that is found in Vista and Windows 7, but not in XP, even with the last service pack.
While Windows Vista was one of the least successful operating systems Microsoft has released (from an adoption standpoint), Windows 7 has become the fastest selling OS ever.