Microsoft could now face more anti-trust complaints after the software giant failed to win its appeal against Commission rulings on Windows XP.
Analysts are particularly concerned that its latest operating system, Vista, could be the next target.
Last year, Microsoft faced complaints from rivals about the way the pending Vista operating system handled search, antivirus security and certain document formats, among other things.
European Union antitrust chief Neelie Kroes had indicated that Vista could limit customers' choices and competitors' ability to develop programs that run smoothly on Windows.
Kroes is not saying what her plans are about Vista, although some analysts think that Microsoft made enough changes to the operating system to ward off further EU scrutiny.