Where now for Microsoft?

Bill Gates' retirement from Microsoft Corp marks the end of an era for the software giant, thrusting CEO Steve Ballmer into the spotlight during one of the most challenging periods in the company's history.

The departure of Gates, who will remain the company's non-executive chairman, coincides with an escalating rivalry with Google Inc and other competitors who are using the Internet to chip away at Microsoft's software dominance.

With Gates taking a step back, the weight of Microsoft's future falls squarely on Ballmer's shoulders.