If you find a file named r3hook.dll on your computer, it might come from one of two sources. It’s most likely a Kaspersky Anti-Virus Ring 3 Hooker from Kaspersky Lab. A hook is a techie term for a piece of software that grabs a process outside its usual path to check it or add to it.
If you don’t have Kaspersky Anti-Virus installed on your system, this may be from Realsoft3D, a modeling and raytracing program created by Realsoft Oy. This is also a hook type of file, but it’s used in the program to make special purpose image formats.
In either case, an error with this file may disable a piece of software on your computer, and you will want to fix it. You can reinstall the affected program, or just start by trying to clean your registry.