DLL hell

Refers to conflicts with Windows DLLs when the wrong one is installed. A Windows DLL is an extension to the operating system that is shared by any application that calls it. Once a DLL is opened, all programs use that same instance of the DLL when they need its functions. Microsoft is continually adding features to its DLLs, and in order for an application vendor to utilize those features and ensure that the latest DLL is available, it installs the latest DLL along with the application.

Installation Programs Must Be Smart
As long as the newly installed DLL is replacing an older version of itself, everything generally works. However, installation programs have not always checked version numbers and dates and have replaced a newer DLL with an older one. This means the installation of a new application can foul up an existing application that depended on functions in the DLL that all of a sudden are no longer there. Fortunately, most installation programs do the necessary version checking.

There Was a Reason
One has to remember, however, that this flawed system was set up when 4MB of RAM was the norm and having two instances of the same DLL open was wasteful of precious memory. Today it is a moot point when 512MB costs less than the 4MB did.

Improvements Have Been Made
Windows 2000 and XP are designed to let software developers install required DLLs in their own application folders and mark them as "not" sharable. Thus, even if a DLL by the same name is already open and in use, the application will use its own version of the DLL instead. These later operating systems also monitor DLLs being replaced and may prevent an erroneous replacement. See DLL.