Steve Ballmer has been CEO at Microsoft Corp for eight years, but he will finally get to move into the corner office vacated by Bill Gates, the college friend who brought him to the company nearly three decades ago.
The pressure of leading the world's largest software maker will only escalate in the wake of a bungled attempt to acquire Yahoo Inc, a move that forced the Web pioneer into the waiting arms of Microsoft's archrival, Google Inc.
Adding fuel to the fire has been a lukewarm reception by customers for the company's flagship product, Windows Vista.
"The pressure is certainly on," said Alan Davis, analyst at investment firm D.A. Davidson.
For the first time in his career, the 52-year-old Ballmer, whose public histrionics often overshadow a sharp intellect and a gift for numbers, must shoulder the weight of Microsoft's future without Gates, who stepped down on Friday from the company he co-founded to focus on philanthropy.
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Their partnership was forged at Harvard University, where the pair formed an unlikely friendship: Gates, the middle child of a prominent Seattle family, and Ballmer, a Detroit native whose parents never went to college.
They both lived in a dormitory full of "anti-social math types," according to Gates. Ballmer, outgoing and involved in many social clubs on campus, seemed to be a study in contrast to the aloof Gates, who preferred all-night programming sessions and poker games.
However, the pair shared a love of math and bonded over their reputations as energetic guys. To this day, they still engage each other in numbers games, calling it "math camp."
After college, Ballmer went to work at Procter & Gamble Co, sharing an office with current General Electric Co CEO Jeffrey Immelt, who has said the two disliked a common boss and would pass the days playing garbage-can basketball.
Ballmer spent a year at Stanford University business school before Gates persuaded him to drop out and become Microsoft's first business manager. A month after joining, he found it was running behind on orders and its engineers were overworked.
"I decided to quit," Ballmer said at an employee event to mark Gates' last day at Microsoft. "I said, 'Jeez, I just dropped out of business school to come to a 30-person company as the bookkeeper'."
Gates persuaded Ballmer to stay at the company over dinner, explaining Microsoft's ambitious vision: to place a computer on every desk and in every home.
Microsoft executives talk about Ballmer's ability to digest large chunks of data, while carefully probing business proposals for weaknesses in logic or reasoning.
Ballmer's sales and marketing prowess complemented Gates' technical acumen as Microsoft grew from a fledgling start-up into a world-beating software company.
He worked up the ranks, becoming Microsoft's president in 1998 and replacing Gates as CEO in 2000. Ballmer is Microsoft's second-biggest shareholder after Gates with a 4.3 percent stake in the company, valued at more than $11 billion.
Michael Silver, analyst at research firm Gartner, describes Ballmer's management style as "scary," but credits him for being a good listener to the needs of his customers.
"Steve's a bright, tough guy and a good marketeer," said Silver. "His personality can be very imposing."
Ballmer often grabs headlines with sharply worded jabs at competitors. He once called free Linux software "a cancer" and dismissed Web search leader Google as "a one-trick pony."
His exuberance for all-things Microsoft has also earned him viral video fame on par with lonelygirl15 or Obama Girl. Video of Ballmer's enthusiastic support for software developers has been viewed more than 1 million times on YouTube, a performance that earned him the unflattering nickname of "Monkey Boy."
"He was always the foil to Gates," said Mary Jo Foley, author of "Microsoft 2.0: How Microsoft Plans to Stay Relevant in the Post-Gates Era."
"Gates is such a serious, plodding, methodical guy and Ballmer knew that to be part of the dynamic duo with Bill, he needed to be the opposite."