REDMOND, Washington (Reuters) - Bill Gates said a teary goodbye on Friday to Microsoft Corp, the software maker he built into the world's most valuable technology company based on the ambitious goal of placing a computer on every desk and in every home.

He leaves Microsoft, which he co-founded with childhood friend Paul Allen in 1975, to focus on his philanthropic organization, the Bill & Melinda Gates Foundation, the world's largest charity, funded in part by his vast fortune.

At an employee event at Microsoft's scenic headquarters campus here, Gates joined Chief Executive Steve Ballmer on stage to deliver a short speech and field questions from employees.

"There won't be a day in my life when I won't be thinking about Microsoft, the great things that we're doing and wanting to help," said Gates, who wiped away tears as the group of employees rose to give him a standing ovation.

Ballmer, a Harvard University classmate who joined Microsoft at Gates' behest, got choked up as he tried to describe Gates' impact on the company and society at large.

"There's no way to say thanks to Bill. Bill's the founder. Bill's the leader," said Ballmer. "This is Bill's baby."

Gates will leave behind a life's work developing software to devote energy to finding new vaccines or to microfinance projects in the developing world. He will remain chairman of Microsoft and work on special technology projects.

Ballmer spoke about how he contemplated quitting Microsoft a month after joining the company and return to Stanford University business school. Bill passionately implored him to stay and laid out the vision of the company.

"This is what Bill said to me. 'You don't get it. You don't get it. We are going to put a computer on every desk and in every home,'" said Ballmer.

There are currently more than one billion PCs worldwide, according to research firm IDC.

Once the world's richest man, Gates' personal fortune has been estimated at about $58 billion, according to Forbes Magazine. He has slipped to third place, behind investor and good friend Warren Buffett and Mexican telecoms tycoon Carlos Slim.

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