You just can't separate sounds.There are somes things that you can do, for exemple: cut the bass in a sound, to put your own bass, but the more you try to change the sound, the more it will sound weird or unatural.
Changing the tempo is easy. You can do it in alot of wave editor (like ce). Or you can strech it in real time in some programs like acid, ableton's live or (i'm not really sure about that) cubase sx. If you got a drum loop or a musical loop that got strong attacks, you can slice it in recycle and make a .rex of it than if you load the sound in a compatible program (most of the sequencing programs) all the slices will snap on the right place they suppose to be. You can just change the tempo and all the each slice will be on time. They won't be strech tho, so if you slow down the beat you'll hear space between slices. I think some hardware sampler accept .rex but I'n not shure. Anybody knows?