REGSVR32

Register or unregister a DLL.

Syntax
      REGSVR32 [/U] [/S] [/N] /I:[CommandLine] DLL_Name

Key
   /u          Unregister Server.

   /s          Silent, do not display dialogue boxes.

   /i          Call DllInstall to register the DLL.
               (when used with /u, it calls dll uninstall.)

   /n          Do not call DllRegisterServer, you must use this option with /i. 

  CommandLine  An optional command line for DllInstall

   /c          Console output (old versions only).

To tell if a DLL is already registered run REGSVR32 dllname.dll from a NON elevated prompt, if the file is already registered this will exit with no message. If it is not registered it will throw an error due to the lack of permissions/elevation.

Examples

Unregister (Disable) CAB file viewer:
REGSVR32 /u C:\Windows\System32\cabview.dll

Register (enable) CAB file viewer:
REGSVR32 cabview.dll

Register DAO 3.6 (Data Access Objects):

REGSVR32 "C:\Program Files\Common Files\Microsoft Shared\DAO\DAO360.DLL"

“I was planning to go into architecture. But when I arrived, architecture was filled up. Acting was right next to it. So I signed up for acting instead” ~ Tom Selleck (on his college registration experience)

Related:

Delsrv - unregister a service with the Services Control Manager. (Win 2K ResKit)
MSIEXEC - Microsoft Windows Installer.
RUNDLL32 - Run a DLL command.
Q249873 - Regsvr32 usage and error messages.