Bill Gates is spending his last day at Microsoft Friday before turning his attention full time to philanthropy after decades spent building the US software colossus.

The Microsoft co-founder, known for his boyish face and nerdy manner, will spend his final day at the software giant he helped found over 30 years ago.

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

After decades devoted to Microsoft, Gates plans to turn his attention full time to the philanthropic Bill & 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’s job as chief software architect is being handled by Ray Ozzie.

Craig Mundie inherited Gates’s chief research and strategy officer duties, while former Harvard University classmate Steve Ballmer is chief executive officer of the Redmond, Washington-based software firm.

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