Microsoft is facing not one but two antitrust investigations by the European Commission.

The first case will look at whether the computing giant unfairly tied its Explorer browser to the Windows operating system.

This was the allegation by Norwegian developer Opera, which took its complaint to the EC in December.

As we reported last year, Opera accused Microsoft of illegally tying Internet Explorer to Windows operating systems and not following "fundamental and open" standards for how web browsers render pages.

Opera wants the EC to force Microsoft to begin offering versions of Windows without IE installed and to make the browser more standards-compliant.

The second case will investigate the interoperability of Microsoft software with rival packages.

The software that will be scrutinised will include Office 2007, the .NET Framework, and some of Microsoft's server products.

Microsoft has said that it will cooperate with the enquiries.

Meanwhile, the EC has released a statement: "This initiation of proceedings does not imply that the Commission has proof of an infringement", it said.

"It only signifies that the Commission will further investigate the case as a matter of priority."

Only in October, Microsoft agreed to put measures in place to comply with the 2004 EC ruling that it had broken EU competition rules - a ruling against which it had appealed and lost.

Microsoft vowed to give third party program developers access to information that will allow them to make systems interoperable with Windows, which was something it had been accused of obstructing.

It also said that it would cut the fees charged for such information.

The ruling also included a fine of close to 500 million euros (£380m).