The messages are from the "playingnow" plugin. On launch, it tries to register these hotkeys but cannot, because, presumably, another application has bagged them already.
You could find that other application and see if it's hotkeys are configurable and change them to something else, or if you don't need them, turn them off....
or...
When you get a 'playingnow' popup, right click on it, hover over "Options" then click on "Edit ini"
Notepad should then open with the plugin's config file loaded.
Look for the second section from the top, called "POPUP SECTION", and there you should see the two conflicting hotkey combinations. Change them to something that doesn't clash, save the changes, then restart MC to verify that the conflict has been fixed. (I changed mine to Win+A and Ctrl+Win+A respectively)
-marko.
