The EC antitrust case is about Microsoft being anti-competitive. They say that they are not allowing competing products to work together with Microsoft products.
It's claimed that Microsoft are using their monopoly of the desktop operating system market to try and also take the server market (with Windows Server) and the media player market (with Windows Media Player).
Microsoft Corporation broke European Union competition law by leveraging its near monopoly in the market for PC operating systems (OS) onto the markets for work group server operating systems and for media players.
Now the EC are saying that Microsoft have not done enough to allow competitors' products to 'talk' to Windows, and are accusing MS of 'dragging its heels' over the issue.
Considering that in Europe, Microsoft have already been forced to release Windows XP 'N' versions, without Windows Media Player bundled, it will be interesting to see how much more Europe will force Microsoft to do.