Developers can download the tool from msdn.microsoft.com/xna to develop games for Windows XP and Windows Vista, at no charge (for now). The games built on Windows can be migrated to the Xbox 360 with the XNA Creators Club subscription for $99 a year, or a four-month trial cost of $49, opening up retail console game development to anyone for the first time.
More info here: http://msdn.microsoft.com/xna