A couple notes:
There is no soft mod for the 360, so I'm not sure what pepito is referring to. He might be referring to DVD-ROM firmware, but that's not really a soft mod anymore for most drives. Dual boot is still what you want.
If you have any basic questions related directly to this topic, please keep them confined to this topic. Otherwise, do feel free to open new topics as that will make it easier on people searching for such information in the future.
Also, dual boot on slim is possible. As far as I can tell, as of this spring, it is supported on all models. (If I'm wrong about that, pepito, please provide us with the correct information.)