Go to the Spybot directory
Open command prompt admin
enter SDFiles.exe /register

To remove from menu right click option

Actually, the final 1.6 code is now using an executable (SDFiles.exe) which 
gives us a nice stand-alone interface instead of the previous method which 
required manually registering SDFiles.dll and only worked within the file/folder 
properties tabs.

 So no DLL's to unregister.  AFAIK, the only thing you 
need to do to remove the "Scan using Spybot-Search Destroy" context 
menu entry is to remove the associated Registry
 
HKEY_LOCAL_MACHINE\SOFTWARE\Classes\*\shell\sdfiles
HKEY_LOCAL_MACHINE\SOFTWARE\Classes\Folder\shell\sdfiles

 At least that worked when I tried it.