Well, if the episodes on your computer are anything that can be played by windows media player (to be blunt) then you can use Windows Movie Maker (if you have XP). I don't know about any other programs you can use, I think Adobe had some kind of movie maker thing though.
I think you're referring to Adobe Premier, but that's like using a supercomputer as a calculator. Windows Movie Maker is much faster, simpler and free. Simply open it, Import your episode(s), drag-and-drop the clips you want and then Export as a .wmv. It's pretty much the easiest thing you can do in terms of video editing.
If you convert .rmvb to .avi the original .rmvb file should most likely still be there. Also there is .rmvb to .avi converters out there, I've used a few of them, and none of them have a problem with the outcome of the quality. The trick is to knowing how to properly convert it with the right parameters.