I have a similar setup with three TVs (with one HD), but I have a Windows XP MCE on a non-HD box. I just use two different outputs in the MCE setup, where the Coax output goes to the non-HD TV and the S-video and audio cables go to the Windows XP MCE system. That way, I can record programs using MCE and it works great. Keep in mind that the MCE PC and the TV will display the same program/channel.
Rob.