There are situations where you need to expand a DLL file or any other system file from the Windows setup CD-ROM, in order to replace a corrupt or missing file in the system. Here are the instructions to expand a file from the source CD.

Note: If you have a Service Pack installed, you need to extract the latest copy of the DLL. Your base Windows setup CD (without any Service Packs included) will not help if you already have a Service Pack installed in your system. Therefore, make sure that you're extracting the system file from a slipstreamed Windows setup CD-ROM (SP2 integration), or get the updated files from %Windir%\ServicePackFiles\i386 folder.

Dynamic-link library (also written without the hyphen), or DLL, is Microsoft's implementation of the shared library concept in the Microsoft Windows and OS/2 operating systems. These libraries usually have the file extension DLL, OCX (for libraries containing ActiveX controls), or DRV (for legacy system drivers).

The file formats for DLLs are the same as for Windows EXE files — that is, Portable Executable (PE) for 32-bit Windows, and New Executable (NE) for 16-bit Windows. As with EXEs, DLLs can contain code, data, and resources, in any combination.

In the broader sense of the term, any data file with the same file format can be called a resource DLL. Examples of such DLLs include icon libraries, sometimes having the extension ICL, and font files, having the extensions FON and FOT.