At the Web 2.0 Summit two weeks ago, Yahoo CEO Jerry Yang was asked if he was the right guy to lead the battered Internet portal. Yang dodged the question by defending his passion for the company he co-founded 13 years ago. “I didn’t make the decision of being the CEO lightly,” he said. “I wanted to make a change at Yahoo that I believe I can make….That’s a dream that I felt I could achieve by being CEO and that’s still the dream today.”

That dream came to an end Monday when Yahoo announced that Yang, 40, will step down as CEO and return to his former role as “Chief Yahoo.” The company’s board said it has hired headhunter Heidrick & Struggles to find a replacement.

Yang has come under fire for his inability to turnaround the company in his past 17 months as CEO. During his short tenure, Yahoo (YHOO) has had two major rounds of layoffs and has seen its search market share shrink significantly while a series of reorganizations led to the departure of senior executives. Yang was heavily criticized by Wall Street and shareholders for failing to reach an agreement to sell the company to Microsoft (MSFT). But the final straw for Yang came when Google (GOOG) pulled out of a controversial ad agreement earlier this month that would have boosted Yahoo’s revenues by hundreds of millions of dollars.

“When the board asked me to become CEO and lead the transformation of the company, I did so because it was important to re-envision the business for a different era to drive more effective growth,” said Yang in a statement. “Having set Yahoo! on a new, more open path, the time is right for me to transition the CEO role and our global talent to a new leader. I will continue to focus on global strategy and to do everything I can to help Yahoo realize its full potential and enhance its leading culture of technology and product excellence and innovation.”

When Yang took over, he was widely viewed as the right choice to replace Terry Semel, the previous CEO from Hollywood who spent six years molding Yahoo into a media company. Yang promised change in the first 100 days as CEO, declaring there would be “no sacred cows.” But 100 days came and went. So did the next 400 days. Frustrated investors have seen Yahoo’s shares drop 62% in value since Yang took over in mid-June 2007. While Semel never had Yang’s geek cred, he did manage to drive Yahoo’s stock price up to an eight-year peak of $43.21 in January 2006. Yahoo’s shares closed at $10.63 on Monday.

Yang has admitted his legacy may forever linked to the debacle with the Microsoft takeover. Yahoo’s board and management team quickly turned down Microsoft’s original offer to acquire Yahoo for $31-a-share in February. The two parties spent six months trying to negotiate a deal. “As a CEO, my job is to find the right path for Yahoo,” Yang said at the Web 2.0 conference. Not getting the deal done “is something that I’ll be labeled with.”