Using windows 7 you should be able to start watching a show that is currently recording on another PC. The only issue is that you will only be able to watch up to the point in the recording where the recording was at the time when you started watching it. So if a show had been recording for 20 minutes when you started watching it, playback will stop at the 20 minute mark. The only way I have found to get around this is to stop playback prior to the 20 minute mark (in this example), and then resume playback. This will now extend your playback time up until that time in the recording process. Still using this example, if you forget to stop, and resume, and you get to the current end, play it again, and then jump to the 20 minute mark by pressing 2, 0, play. This will take you to the 20 minute mark.
Are you using Windows 7?
Do you have your Recorded TV in your Library?