He also was instrumental in growing Microsoft from a company that had fewer than 40 employees and $12 million in annual revenue when he came aboard. In 2012, Microsoft had 94,000 employees and $74 billion in annual revenue.
When he took to the stage to extol Microsoft, Ballmer often acted more like a crazed cheerleader than the chief executive of an influential company. In one presentation that eventually became a viral sensation on the Internet, Ballmer bounded across the stage, jumping up and down while yelping and imploring the audience to stand up, before breathlessly proclaiming, "I LOVE THIS COMPANY!"
But Microsoft enjoyed its greatest success with Gates at the helm and Ballmer as his sidekick.
Gates turned over the reins to Ballmer in January 2000 in what was considered to be a surprise move, because Ballmer had been considered more of a numbers and sales specialist, not a technology specialist.
The CEO change came just a few weeks after Microsoft's stock hit a record high of nearly $60, on a split-adjusted basis.
Janney Capital Markets analyst Yun Kim said that while the stock should get a boost from the news, investors shouldn't get too excited, because the company itself won't change overnight.
Kim said the new CEO, who will likely come from outside the company, faces the "daunting task" of making Windows relevant amid the continued consumer shift away from PCs.
Ted Schadler, an analyst at Forrester, said that while some may try to write its obituary, Microsoft still has some reliable cash cows. Its software, like Windows and Office, is still popular. So is Microsoft Enterprise, which helps big companies run databases, and the Xbox gaming system. Schadler noted that about 70 percent of business email is still sent on Microsoft software.
Part of Microsoft's downfall stemmed from the bursting of a technology bubble that helped inflate the company's stock just before Ballmer took over.