Microsoft's plans to end support for Windows XP, believed to be the dominant computer operating system in K-12 education, could pose big technological and financial challenges for districts nationwide--issues that many school systems have yet to confront. The giant software company has made it clear for years that it plans to stop supporting XP next year, and it has been urging districts, as well as businesses and other customers, to upgrade to Windows 7 or 8. Technology experts who work with districts say many of them have yet to take that advice, or to buy other up-to-date operating systems, either because of tight budgets or a reluctance to disrupt a technology that is familiar to many teachers, students, and administrators. But now the need to upgrade has become more urgent.