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How do you tell the difference between Checked Build Windows or sometime called Debug Build and Free Build or commonly known as Retail Build? Here’s the distinction of the two:

The checked build (or debug build)
The purpose of the checked build of Microsoft Windows is to make identifying and diagnosing operating-system-level problems easier. The checked build differs from the free build in the following ways:
Many compiler optimizations (such as stack frame elimination) are disabled in the checked build. Disabling such optimizations makes it easier to understand disassembled machine instructions, and therefore it is easier to trace the cause of problems in system software.
The checked build enables a large number of debugging checks in the operating system code and system-provided drivers. This helps the checked build identify internal inconsistencies and problems as soon as they occur.

The free build (or retail build)
The free build of Microsoft Windows is used in production environments. The free build of the operating system is built with full compiler optimizations. When the free build discovers correctable problems, it continues to run.
Distribution media containing the free build of the operating system do not have any special labels — in other words, the CD containing the free build will just be labeled with the Windows version name, and no reference to the type of build.

To read more, click on this link:

http://msdn2.microsoft.com/en-us/library/ms792442.aspx