The European Commission is launching two new anti-competition investigations against US computer giant Microsoft.
Microsoft agreed to comply with the EU's previous ruling in October
The first will look at whether Microsoft unfairly ties its Explorer internet browser to its Windows operating system.
In a parallel probe, the Commission will look at the interoperability of Microsoft software with rival products.
The firm said it would cooperate with the latest enquiry. In October, the EU ruled the firm broke anti-trust rules.
"This initiation of proceedings does not imply that the Commission has proof of an infringement," said the Commission in a statement.
"It only signifies that the Commission will further investigate the case as a matter of priority."
The latest investigation by the Commission comes after two complaints, one by Norwegian company Opera, the other by pan-European software-makers group European Committee for Interoperable Systems.
The move by the Commission's competition officials will stoke the flames of a long-running battle between Brussels and Microsoft that many thought had finally died down last year.
In October 2007, Microsoft agreed to comply with the Commission's 2004 ruling that it broke European Union competition laws.
The US firm's move came after it lost an appeal against the verdict, which included a fine of almost 500m euros ($745m; £380m).
In that case, Microsoft was judged to have shut out rivals from its Windows operating system to gain a larger share of the market for web servers.
Microsoft pledged in October to give third party program developers access to information that will allow them to make systems interoperable with Windows.
It also said it would substantially cut the fees it charges for such data.