A big jump in Microsoft stock helped lift the Dow Friday. Microsoft had its biggest gain in four years after chief executive Steve Ballmer said he will retire. The Dow closed down for the week and is 3.2 percent lower for the month. Stocks have sagged in August on concerns that the Fed will start to ratchet back its economic stimulus. Stocks had climbed to record highs at the start of the month. The market may become more volatile in the coming weeks as traders anticipate the timing of the Fed’s move, said Randy Frederick at the Schwab Center for Financial Research. Investors will also start to follow the debt ceiling debate in Washington.