When adding a new file to the project, you can specify that it's an ActiveX / COM library, and it needs to be registered in the virtual registry when the application starts.

For example, suppose, your application plays flash movies. Then you simply add the flash.ocx file and specify that this file is virtual, set the "register in virtual registry" checkbox -- and your application no longer depends on whether or not the client computer has a flash player installed.

Why does that happen? The thing is that when the application starts, it calls the function for registering the specified ActiveX (yes, it can be a virtual file actually!); at the same time, for the time the function is running a special flag remains set, and all changes are saved in the virtual environment. In other words, the real registry doesn't get modified, and the rest of the application's code runs just as if the ActiveX was actually installed in the system.

And what's especially important is that the modification of the original exe isn't required.