The computer has to be turned on in order to do this. You can have click a setting when initially setting up windows media center on your 360 so that as soon as the power is turned on your computer, you can access all of the shared files. However you can turn off the display, tone down the performance options so that you conserve energy. On a side note, you could use an external hard drive to store your files from your computer and then simply plug the usb into your xbox 360 when you wish to view files on there. Very simple and effective.