Microsoft could have escaped the wrath of European Union antitrust regulators and the risk of potential fines in the billions if it had paid attention to an exchange on its own support site more than a year ago.

The back-and-forth between a user and a company support engineer on the Microsoft Answers website occurred in March 2011, just weeks after Microsoft shipped Windows 7 Service Pack 1 (SP1).

That upgrade and its release date -- late February 2011 -- are the crux of the new investigation by the European Commission.

On Tuesday, the commission accused Microsoft of omitting a screen that gives users easier access to rivals' Web browsers.

The browser choice was the result of a deal the Redmond, Wash., developer struck with EU regulators in 2009 after critics claimed Microsoft's Internet Explorer had an unfair advantage because it was bundled with the Windows operating system.

Joaquin Almunia, the commission official in charge of antitrust enforcement, said Microsoft had broken the three-year-old agreement. "It appears that since the launch of Windows 7 Service Pack 1, the choice screen has not been displayed," Almunia said at a Tuesday press conference.

Almunia vowed to use "the legal instruments with all my capacity to deter and punish" violators like Microsoft.

The commission could charge Microsoft a fine of up to 10% of its revenue during the time that the browser choice screen was not shown to Windows 7 SP1 users. Depending on how the agency calculates that revenue, fines could reach more than $8 billion. It's unlikely that a fine would be that large, however. The largest-ever set by the commission was the $1.3 billion it imposed on Intel in 2009.