Shoutbox

This is only a workaround.

Open Windows Media Player, from its menu, go View>Add-ons>Other, here untick Windows Live Messenger Music Add-on. Now try to uninstall it.

Or try manually deleting that key from registry. you can access Registry by Start>Run>regedit . But modifying registry is risky and manually modifying it may cause your programs or computer not to start properly or never. Do it at your own risk.