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.

Microsoft could have avoided the new investigation and possible sanctions if it had heeded the tip provided by a Windows user identified only as "RichardD."

In a question posted to Microsoft Answers on March 28, 2011, just five weeks after Windows 7 SP1's debut, RichardD asked why he was unable to find the browser choice file on his PC.

"I have installed SP1 and I do not see the options for the browser choice," said RichardD. "I have installed SP1 on an English machine with its region set to France. I have also tried on a French machine, with France as the region. Is KB976002 included in SP1, or is it implemented in a different way?"

KB976002 refers to the Microsoft support document that explained the browser ballot.

A day later, Afzal Taher, who identified himself as a "Microsoft Support Engineer," replied to RichardD, telling him, "Windows 7 SP1 contains all the previous Windows Updates (which includes browser choice update as well)."

That didn't satisfy RichardD, who had already looked at the list of individual updates rolled into SP1, and had confirmed that the browser choice update was supposed to be in the upgrade.

"I understand that it is included in the service pack, per the documentation. However is it really installed with Service Pack 1?" asked RichardD. "I have a clean Windows 7 RTM, with its region set to France and used Windows Update to install SP1. There is no BrowserChoice.exe in the system32 folder. How can I verify that it is installed with SP1?"

Taher, the Microsoft support engineer, never replied to RichardD's last question.

It's possible that had Taher pushed RichardD's query about the missing browser choice update up the chain of command, or investigated further himself, Microsoft would have realized that it had bungled long before the commission got involved.

Tuesday, Microsoft apologized and characterized the omission of the browser ballot as a "technical error."

"Unfortunately, the engineering team responsible for maintenance of this code did not realize that it needed to update the detection logic for the BCS [browser choice screen] software when Windows 7 SP1 was released last year," the company said in a statement Tuesday. "As a result of this error, new PCs with Windows 7 SP1 did not receive the BCS software as they should have."

In the same statement, the company said it had only learned of the blunder when it was recently told by the commission that others had reported the ballot was not showing up on some PCs.

Microsoft declined to comment on the browser choice-related exchange found on its support site.