SAN FRANCISCO- Macintosh computer fanatics wont have Bill Gates to kick around anymore. The Microsoft co-founder whose boyish face and nerdy manner epitomizes the US software colossus spends his last day at the office on Friday.

Paul Allen, who teamed with Gates to start Microsoft in a garage in 1975, will be among those “roasting” his childhood friend at a gala dinner affair Friday night.

After decades devoted to Microsoft, Gates turns his attention full time to the philanthropic Bill and Melinda Gates Foundation he established with his wife.

Gates leaves Microsoft to wrestle with a fast-changing computer era and growing challenges from Internet juggernaut Google and longtime rival Apple, which makes Macintosh computers.

Three people will essentially fill the void Gates leaves behind at Microsoft. Gates job as chief software architect is being handled by Ray Ozzie.

Craig Mundie inherited Gates chief research and strategy officer duties, while former Harvard University classmate Steve Ballmer is chief executive officer at the software colossus based in Redmond, Washington.

Gates remains chairman of the Microsoft board of directors and its largest shareholder.

Microsoft is losing Gates at a time when “cloud computing” is shaking the packaged software foundation on which the companys fortune is built.

Meanwhile, Microsofts Windows Vista operating system released in January of 2007 has flopped with customers, many of whom are clinging to its predecessor Windows XP.

Microsoft failed in a recent bid to buy Yahoo for nearly 50 billion dollars in order to combine online resources to better battle Google in the booming Internet search and advertising.

Gates is expected to maintain the ears of Microsoft leaders. He bequeathed them a legacy that includes focusing in-house teams on what Gates expects to be major technology trends, such as an “Internet tidal wave.”

“At some point the firm has to take the essence of what made Bill Gates successful and make sure that is preserved,” said analyst Rob Enderle of Enderle Group in Silicon Valley.