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 blue boxes, will require what Microsoft calls a “Custom Install,” also known as a “clean install”–a procedure Microsoft doesn’t even refer to as an “upgrade.” For most average, nontechie consumers whose PCs have a single hard disk, that will require a tedious, painful process with the following steps: Temporarily relocating your personal files to an external drive or other computer, wiping your hard drive clean, then installing Windows 7, then moving your personal files back, then re-installing all of your programs from their original disks or download files, then reinstalling all of their updates and patches that may have been issued since the original installation files were released.

Microsoft will provide a free “Easy Transfer” program to assist in this process, but this software won’t transfer your programs, only your personal files and settings.

Microsoft made some mistakes while making this chart. Like there is nothing like Windows vista started edition 64 bit !! Is microsoft going to launch that as well with windows 7
Also according to chart you can options are given for going to 64 bit to 32 bit and vice verca, which is also not possible as far as i know..