Apologies Mack, that was my shorthand description, and it does not fully describe the argument.
As I understand it the point argued by Opera/Mozilla/Google is that the (technically unnecessary) integration of IE into the Windows OS is and always was a deliberate attempt to stifle competition.
This involves such things as the blue E always appearing on the desktop, Windows Update not working with other browsers, and the questionable notion that a web browser is not an additional application but a feature of the operating system.
The result is that many people do not even know that they are using something called a browser, let alone what its name is, or that there are alternatives available - and even if they are aware, they must have IE for the OS to function, so they might as well not bother with other browsers.
There is also the question of OEM distributors - they commonly offer the Windows OS with additional or custom software, but they can't currently replace IE with another browser of their choice, which some might say is anti-competitive.
I am no expert on monopoly legislation but Microsoft was found to have acted illegally by the US courts insofar as their practises were designed to thwart competition, and I believe that the EU courts have the power to order them to change.
Whether they will do so remains to be seen, but if Google is allowed to join the proceedings (as another browser manufacturer) then both sides will have access to as many lawyers as they want, and the contest will be between two 800 lb gorillas rather than David and Goliath.
I only joined this debate because the opening response was so obviously wide of the mark - there seems to be a widely held misconception that the case is simply about bundling a web browser with the OS, which I doubt that anyone objects to.
It is about monopoly and competition, and the obscure laws that govern them.