Yesterday the European Commission announced that it had decided to fine Microsoft $731 million for breaching antitrust commitments. It could have been as much as $7.4 billion, but Microsoft’s co-operation in the EC investigation went a long way to keeping the fine relatively low.The specific reason a fine be paid is due to Microsoft removing the Browser Choice Screen from copies of Windows 7 back in February 2011. It then remained missing for 14 months. Microsoft claimed it was an accident and side effect of pushing out Windows 7 Service Pack 1 and they hadn’t noticed it was missing.One question that wasn’t answered yesterday as part of the announcement was how the commission came to discover the browser selection screen had disappeared. But the reason has now been revealed via a number of sources who wish to remain anonymous.According to the Financial Times, both Google and Opera complained to the commission about the screen’s removal, which prompted the investigation. Not only that, but both companies are thought to have helped the commission during the investigation which lasted 8 months.Opera is the company that originally kicked off the complaints against Microsoft shipping Internet Explorer with Windows. Google’s involvement is a new development, though, and demonstrates how important the web browser market is to the company as well as Microsoft’s clear position as a major competitor to them.Opera has commented that the company is happy to see the commission taking action, Google is declining to comment, and so far Microsoft hasn’t said anything about the fine. You can guarantee everyone involved will be continuing to watch Windows carefully every time Microsoft pushes out a new update.