Internet media company RealNetworks has sued Microsoft, accusing it of unfairly monopolising the growing market for digital music and video.
Microsoft's media player has boasted stellar growth
Accusing Microsoft of "predatory conduct", RealNetworks has asked for more than $1bn (£564m) in compensation.
"We believe our business would be substantially larger today if Microsoft were playing by the rules," chief executive Rob Glaser said.
Microsoft has denied the allegation, insisting the market was competitive.
Microsoft pointed out that RealNetworks was the number-one provider of digital media technology, arguing that it was using antitrust law in order to protect and increase its market share.
The RealNetworks lawsuit runs in parallel with an investigation by the European Commission into Microsoft's media-playing software.
Brussels is concerned that Microsoft may have used its dominant share of the computer operating-system market to force users to adopt its media technology.
In its lawsuit, RealNetworks said Microsoft's monopoly meant that every Windows user had to take its media player, "whether they want it or not".
RealNetworks, meanwhile, sells its media player as a
downloadable software product or with a monthly subscription.
As broadband internet access proliferates, downloading and playing video and music is seen as one of the main growth areas.
The core of RealNetworks' argument is that Microsoft has been able to achieve extraordinary growth.
Since launching its media player in 1997, Microsoft is now nearly neck-and-neck with RealNetworks, and by some measures - notably in the US market - even excels.