DLL is a file format used by Microsoft Windows that stores a lot of resources. Commonly DLL files are used to store resources that are not to be seen. And some of the settings and codes are also stored in DLL files. You can see a lot of DLLs in the System32 directory located at the Windows system folder. Most of the operating systems settings and resources are stored in that DLLs. So when you want to edit some of that data you have to decompile that and view the resources.
We can make this happen easily by using a resource decompiler like PE Explorer. These programs can decompile few of the compiled file formats as EXE, DLL, CPL, OCX etc. If we get the shell32.dll as an example, you can extract or change all the system icons used in windows through one of these softwares. But this is an illegal process that called as resource hacking.
We can extract most of the data inside a DLL by using PE Explorer. But we can’t decompile all the DLL files. Some files are decompiling protected and some are containing only a lot of blocks of codes. We can understand some of those codes via disassembling option in the software.
PE Explore is a software that developed by Heaventools. It is an commercial product. You can view some of the screenshots from here (http://www.heaventools.com/scrshots.htm) and you can download and try the program or you can directly buy the product.
*This instructions are only for the educational purposes. So please do not use these programs for illegal activities.