Responding to an earlier posting here, Sylar is predispositioned to use powers. We learned that in S1 when Mohinder's father tracked down Gabriel Gray because he had the genetic marker common in those who are the next step in human evolution. Gabriel showed no abilities because he had yet to to "read" somebody's brain. But his DNA enabled to acquire the knowledge of powers and how to use them.
As for Arthur "stealing" powers, maybe it isn't that he is stealing the powers, but is using his mental abilities to "block" a person from using them. (And if that is the case, it may be likely that Matt can help Peter restore his powers.) We now know Arthur has a significant power in altering one's memory, as he did with Angela before Linderman healed her. He could theoretically be doing the same thing to Peter. Mind you, this doesn't explain why Adam crumpled into dust when Arthur took his regenerative powers so I'm probably way off on this one. UNLESS, in the process of "blocking" the regenerative abilities of Adam, his body became unaware of what it had to do to maintain its youthful state and reverted to its real age. I guess we'll have to wait and see if they explain this one. If we knew all the answers ahead of time, there would be no reason to watch the show.
I still think Arthur learned how to acquire powers from the Sylar videos, however. I don't believe Adam knew what Arthur was going to do when he was brought to him; otherwise why would Adam approach the bed on his own accord? I think the shock and fear Adam experienced was from seeing someone he believed to be dead, someone quite powerful who would have had a hand in locking Adam away for 30 yuears.

