Symantec Corp. agreed to buy Blue Coat Systems Inc. in a $4.65 billion deal that will give the computer-security company a new portfolio of cyberdefense technologies along with a new chief executive. Blue Coat Chief Executive Greg Clark is set to take that role at Symantec after the deal closes, which is expected to occur before October. Symantec has been searching for a new CEO ever since Michael Brown stepped down in April amid disappointing financial results.

Following a rash of high-profile breaches, computer security is now a major topic of discussion in corporate boardrooms. While Symantec is a pioneer in antivirus software that runs on personal computers, Blue Coat’s domain is the World Wide Web. The company’s technology is used by more than 15,000 companies to block dangerous or inappropriate websites.

Blue Coat is also a seller of security services delivered over the internet, also known as “cloud” computing. Blue Coat had filed for an initial public offering earlier this year, and analysts’ estimates for the value of the company should it have taken that route were in the range of what Symantec is paying.

The deal is Symantec’s largest acquisition since its 2005 purchase of Veritas Software, and the latest in a series of big steps it has taken as Symantec tries to turn around its fortunes in the increasingly competitive market for computer-security products.