Shares of software giant Microsoft (NASDAQ:MSFT) raced as much as 4.7% higher this morning. Microsoft's stock was not only the biggest gainer by far on the Dow Jones Industrial Average (DJINDICES:^DJI), but also the fourth-biggest gainer on the much wider S&P 500 (SNPINDEX:^GSPC) index. That's an impressive move for an industry titan with a bulky $285 billion market cap to push around, especially on a flat day for the Dow. It takes big news to engineer a jump like this one.

The catalyst is activist investor firm ValueAct Capital Management's disclosure of a brand-new $2 billion stake in Microsoft stock. This makes Microsoft the firm's largest holding, according to Bloomberg.

Why is this such a big deal? Well, Microsoft's management troubles have been widely documented, and a change at the top could spark new life in these listless shares. ValueAct specializes in finding turnaround avenues for failing businesses with healthy fundamentals, and Redmond fits that bill to a "T."

This could be the first sign of a major shakeup in Redmond -- and a much needed one, I might add.

ValueAct has been active in the software sector lately. The firm took a 5% stake in multimedia specialist Adobe Systems (NASDAQ:ADBE) in 2011. A year later, Adobe announced a "standstill agreement" with the activists that would prevent ValueAct from running any direct takeover plays but also placed ValueAct partner Kelly Barlow on Adobe's board of directors.

Barlow immediately promised to transform the "iconic" Adobe brand. Adobe shares have crushed the market since ValueAct became involved, and even more so since the standstill agreement was announced in December 2012.

ADBE Chart

ADBE data by YCharts.

ValueAct would love to do the same for Microsoft's even larger brand, and investors seem to like the idea very much. They're voting "yea" with their wallets today.

Fool contributor Anders Bylund holds no position in any company mentioned. Check out Anders' bio and holdings or follow him on Twitter and Google+.

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