Windows and the installed programs uses DLL files (dynamic link libraries, more details about DLLs here) to store functions and codes to use whenever required. Problems to these files may cause system errors.

There are many reasons which result in DLL problems, for example malware infection to these files. UnDLL from ESET allows you to remove virus infected DLL files, which are often used as components of many types of malware such as Agent.CS or Virtumonde.