At least one Microsoft investor isn't happy with chief executive Steve Ballmer's performance. David Einhorn, hedge fund manager for Greenlight Capital, called for the Microsoft chief to step down on Wednesday during a speech at an investor conference. Ballmer's presence is the biggest drag on Microsoft's stock, according to Einhorn. The high-profile investor also blamed Ballmer for wasting billions on research and referred to Bing as a "sinkhole," according to The New York Times. Microsoft's stock is widely considered undervalued at under 10 times its expected earnings, according to Reuters.