Synacor’s stock lost nearly 40 percent of its value Thursday after the Buffalo-based Internet content provider warned that it expects only modest sales and earnings growth this year.

The tepid forecast, coming after Synacor’s fourth-quarter profits also fell short of analyst forecasts, sparked a major sell-off in the company’s shares, which had been one of the hottest stocks in the Buffalo Niagara region last spring and summer. Synacor’s stock tumbled by $1.84, or 38 percent, in heavy trading Thursday to an all-time low of $3, well below its initial public offering price of $5 last February.

“While we remain passionate about our long-term opportunities, we expect 2013 to be transitional,” said Synacor CEO Ronald N. Frankel.

Synacor warned that its sales this year would only grow by about 2 percent – far slower than its 34 percent revenue growth last year – while its adjusted cash flow would improve by only 5 percent, just a fraction of its 53 percent jump in 2012.

Synacor executives said during a conference call that much of the slowdown was due to changes in Microsoft’s new Windows 8 operating system, which made the software-maker’s Bing search engine the default search provider and set its MSN site as the home page on computers using Windows 8.

That change relegated the start pages that Synacor designs for its customers, such as computer-makers Toshiba and Lenovo, along with a handful of cable television providers, to a secondary tab on those computers’ Internet browsers. That hurts Synacor because the company generates revenue every time a subscriber uses the Google search box on the start pages that it designs, while a reduction in page views also hurts Synacor’s advertising sales on those start pages.

“Synacor will face an adjustment period, which we think is reflected in management’s flattish year-to-year revenue guidance,” Rich Tullo, an analyst at Albert Fried & Co., said in a report issued Thursday that reduced his target price on the stock to $6, from $7.

In the longer term, Frankel said, the company still believes that it has strong growth prospects, as its TV Everywhere system gains acceptance. The company also launched a Cloud ID offering that lets Synacor customers access the content from TV Everywhere with the same login that they use on social media sites such as Facebook and Twitter, Frankel said.

In addition, Frankel said Windows 8 could eventually turn into a positive for Synacor through the development of tiles for the main screen of Windows 8 devices that provide a direct link to the startup pages of Synacor’s customers.

During the fourth quarter, Synacor’s profits tumbled by 90 percent, to $790,000, or 3 cents per share, from $7.7 million, or 34 cents per share, a year ago, when earnings were inflated by a $6.1 million income tax benefit. Its adjusted cash flow improved by 30 percent, to $3.5 million, from $2.7 million.

Synacor’s fourth-quarter revenues strengthened by 11 percent, to $32.2 million, from $28.9 million, fueled by a 13 percent increase in revenues from search and display advertising, which accounts for about 84 percent of the company’s total revenues. Subscription-based revenue grew by 4 percent.

But during this year, Synacor warned that its growth would slow markedly. The company said its revenues would be between $122 million and $126 million, up by only about 2 percent at the midpoint from last year’s $122 million. Its adjusted cash flow is expected to be between $11.6 million and $12.6 million, which is about 5 percent more than last year’s $11.6 million at the midpoint of the guidance.

“Guidance was dour,” Tullo said. “We think Synacor shares are dead money for a while, which is unfortunate, as the company is working on initiatives which could be harvested in the second half of 2013.”