(Reuters) - Symantec Corp (SYMC.O), best known for its Norton antivirus software, said Silver Lake Partners had invested $500 million in the company and that the private equity firm would get a board seat.

Also, activist shareholder hedge fund Elliott Management Corp has piled up a large stake in Symantec, becoming one of its largest shareholders, the Wall Street Journal reported late Thursday.

Elliott also supports the Silver Lake investment and other measures announced Thursday, the Journal added, citing sources.

Symantec also announced a $2.3 billion share buyback and declared a special dividend of $4 per share, which would total to a payout of about $2.7 billion.

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The share buyback and special dividend takes Symantec's total capital return program to $5.5 billion, which it expects to complete by March 2017.

Symantec, whose shares were up 9.5 percent at $21 in extended trading on Thursday, also reported better-than-expected revenue and profit for the third quarter.

Silver Lake will buy $500 million worth of Symantec's unsecured convertible notes. The firm's managing partner, Ken Hao, will join Symantec's board.

"Now that Silver Lake is behind Symantec there is a hope that they'll make the right decisions moving forward for a company which has made a lot of bad decisions over the last decade," FBR Capital Markets analyst Daniel Ives said.

Symantec has been facing headwinds as slowing sales of personal computers hurt demand for its security software, which comes bundled with computers.

As the demand for traditional antivirus software falls, the company has been making software that have multiple layers of security, especially for businesses looking to protect themselves against increasingly sophisticated cyber attacks.

Symantec said on Thursday the buyback and dividend payout would be funded through proceeds from the sale of its data storage unit Veritas, Silver Lake's investment as well as additional debt and cash.

The company said it would halve its regular annual dividend to 30 cents per share from next quarter due to the special dividend and lower estimated domestic cash flow after the sale of Veritas.

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Up to Thursday's close of $19.18, the Mountain View, California-based company's stock had lost 25 percent in the past 12 months.

Elliott Management and Symantec could not be reached immediately for comment outside regular business hours.

(Reporting by Abhirup Roy and Rishika Sadam in Bengaluru; Editing by Savio D'Souza and Gopakumar Warrier)