If you put a GameCube disk in your computer can an emulator read it?

No, Dolphin emulator cannot play xbox games only gamecube and wii games. Currently there are no good xbox emulators worth mention. If you want to play xbox ISOs you will have to mod an xbox or play the games legitimately.

However many, xbox games have equivalents on the gamecube or playstation 2 both of which can be emulated well.