I configured my computer as a Microsoft Media Center Keyboard & then adjusted the power button settings within the Harmony software. You can set the “power on” button to Guide or LiveTV, or whatever you want. Also, set the “power off” button to Alt-F4 to close MediaCenter or you could set it to Stop in order to leave MediaCenter open, but stop playback.
I’ve tweaked mine like crazy and I’ve even set up AutoHotKey to create keyboard shortcuts for MyMovies and Netflix. My IR receiver is an iMon VFD & so I was able to reassign the keyboard shortcuts for unused buttons (like Movies, Videos, Radio, etc.) to perform the keyboard shortcuts I created with AutoHotKey. Then I created custom buttons on the Harmony for Netflix and MyMovies to send the unused commands that I remapped within iMon. This may sound confusing, but it’s really not as bad as it sounds. It was pretty easy to setup once I got the hang of AutoHotKey.