It will not replace WMP, in that it will not uninstall WMP from your comp (I don't even think if anything Microsoft is uninstallable from a Windows PC :S). You can choose which player to play the movie on (I think that upon installation you will be asked wether you want the player to take over Videos, so that when you double-click a video, it will open in that player) and no, you can't burn with those players (the players themselves are not sites: they are applications). However, IMO, it is better to just have a standalone application for burning stuff (such as Nero) because it is more reliable and offers more options.

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iMac G4 OSX 10.3.9
RAM 256MB
LW 4.10.5 Basic
ADSL anything from 3 to 8Mbps/around 1024kbps

"Raise your can of Beer on high
And seal your fate forever
Our best years have passed us by
The Golden Age Of Leather"
-Blue Öyster Cult-