DLL

Dynamic Link Library. An executable program that performs some function, but is not launched directly by the user. When needed, they are called for by a running application and loaded to perform a specific function. DLLs are generally written so that their routines are shared by multiple applications at once. Windows uses DLLs as a standard way of linking and sharing functionality. Many DLLs are included in Windows that other programs depend upon. Some applications include their own DLLS, which are typically stored in the application directory.

So when Spyware uses an ActiveX Control to keep reinstalling itself, it is usually a DLL file.