Microsoft and Adobe are at war: they both want to be leaders in a new web software market. Both companies today unveiled new software that crosses into the companies' respective areas. First, there was Adobe's Media Player video tool, and now there's Microsoft's 'Flash killer' Silverlight browser plug-in.

So far, Adobe's Flash standard has pretty much been the standard for online video streaming from websites such as YouTube and MySpace , but now Microsoft wants to break this tradition.

Microsoft's Silverlight plug-in plays media files and runs interactive internet applications. It works with Internet Explorer, Firefox and Safari, thus allowing both PC and Mac users to stream high-definition WMV files as they do with Flash files.

The Silverlight application - which has been under development for more than two years - supports HD video, which Flash doesn't, thus gaining an immediate advantage. It is also optimised for both PC and Macs; there is no difference in runtime and no Microsoft software is needed to run the application.

Microsoft's Tim Sneath lists on his blog 10 reasons why developers might want to use the 2MB Silverlight plug-in instead of Flash - one of which is that there is more to come.

"It supports playback of WMV files on both PC and Macintosh, with many options for interactivity during playback; with just a couple of lines of code, you can provide a platform-neutral way to handle all your movie files," he wrote.

"Silverlight supports full-screen 720p video and offers seamless transitions between full-screen and windowed mode without losing your position in the video (something that media sites are crying out for today)."

Eventually, Microsoft plans to create versions of Silverlight that run on mobile devices and other operating systems.