SAN FRANCISCO – The chief executives of McAfee Inc. and CNet Networks Inc. stepped aside Wednesday to atone for stock-option chicanery that occurred on their watch, expanding the list of business leaders swept up in a messy scandal that is erasing profits and spurring government investigations.

McAfee, a leading maker of anti-virus software, parted ways with George Samenuk, who retired as its chairman and CEO after nearly six years on the job. The Santa Clara, Calif.-based company also fired its president, Kevin Weiss. Wednesday’s purge wasn’t the first at McAfee, which fired its general counsel in May after determining he had been involved in stock-option improprieties.

CNet, which runs a collection of technology news and entertainment Web sites, accepted the resignation of co-founder Shelby Bonnie, who has been running the San Francisco-based company for the past six years.

McAfee named board member Dale Fuller as its interim CEO and president, and it tabbed Charles Robel as its nonexecutive chairman.

CNet promoted Neil Ashe, a company executive since 2002, as its new CEO and appointed Jarl Mohn as nonexecutive chairman.

Both Samenuk and Bonnie expressed regret for the improper accounting of stock options that occurred during their tenures.

McAfee shares gained 71 cents to $26.50 during late afternoon trading on the New York Stock Exchange while CNet shares fell 72 cents, or more than 7 percent, to $9.18 on the Nasdaq Stock Market.

The mishandling of the past stock options awarded to employees will force both McAfee and CNet to wipe out some of their previously reported profits and may result in fines or other penalties if regulators and prosecutors determine that the misleading practices broke laws.

McAfee and CNet are among 130 companies nationwide whose stock-options practices are under government investigation or internal review.

Most of the inquiries are focused on a technique known as “backdating,” which occurs when insiders look back in time for a low point in their company’s stock price so the exercise, or “strike,” price of the options could be set at that ebb.

As more companies have owned up to their backdating problems, they also have been cleaning out their executive suites and boardrooms. Most of the fallout has been centered in Silicon Valley, where stock options first became a staple of compensation packages during the 1990s.