Media Center prevents system standby while playing, but if you pause playback it will allow the computer to sleep. This seems reasonable to me.
Are you saying you want it to stay awake forever while paused?
In audio mode, MC will prevent standby only while audio is playing, it will still go to standby (within minutes if I remember correctly, not 20 as set in power set-up) after the play list finishes, which IMHO is perfect. Unfortunately the same algorithm is playing havoc in Video mode.
I would like the computer to observe my aforementioned time-outs in both Pause and Theatre Menu modes. I believe there is presently a bug in the software that is not resetting the time-outs when a movie is Paused or Stopped after running for more than the computer set time-out which in my case is 20 minutes. I suspect the anomaly is also present in the play back of audio when paused after playing for more than 20 minutes.
While I have your attention, can I propose you consider adding a screen saver during Pause mode! This is a feature of WMC that is a
necessity for plasma viewers that wish to avoid burning their screens.