Can I move ether of one arbitrary account to another one by a contract automatically?

Question

I'd like to move ether from one arbitrary account to another account following a contract. From the execution method, I'd like to send ether. Can I do that?

contract MultiplyContract{
    address public sender;
    address public receiver;
    uint public price;
    function MultiplyContract(
      address _sender,
      address _receiver,
      uint _price
      ){
      sender = _sender;
      receiver = _receiver;
      price = _price;
    }
    function Execution(){
      //sender send ether to receiver//
    }
    function () {
      throw;
    }
}

Answer 1

No. A contract can only spend Ether from its own funds. Being able to sign a transaction for an arbitrary account would entail the contract having access to said account's private key, making it public for everyone.