Microsoft has release a Windows 7 upgrade chart to help user to choose how his existing OS can be upgraded to windows 7.

Common versions of XP and Vista are listed down the left side, and the three versions of Windows 7 are listed across the top.
Note that ONLY those combinations that intersect in a green box saying “In-Place Upgrade” can be upgraded in a simple way that, in Microsoft’s words, “Keeps your files, settings, and programs intact from your current version of Windows.”
All of the others, denoted by …