Pam, it goes like this:
1. Sylar gets stabbed, regenerates, explodes.
2. Future Hiro goes back in time to get Peter to save Claire so that he won't regenerate.
3. Things change. Claire lives. But now Peter explodes. (Peter himself explicitly stated this in the ep, and Sylar confirmed it when they faced each other.) Everyone thinks it's Sylar because Nathan covered it up, but in this (second) timeline, the bomb is really Peter.
4. Since there was still a bomb (and since everyone still thinks it was Sylar), nothing really changes except that Claire gets to live.
5. Present Hiro returns to his own time, armed with new knowledge (except, unfortunately, for the bit about Peter being the bomb instead of Sylar).
