IMO, the most reliable thing to do is to for WMC is this:
-setup WMC on a clean Windows installation,
- make very minimal adjustments (like installing mychannel logos, Ceton Companion services)
-tell your bios to restart the computer automatically in the event of a power outage
-remove the monitor, keyboard, and mouse
-put it in a corner somewhere
-stop messing with it
Then use extenders for live television and DVR. This was reliable enough to setup for my parents. I wouldn't dream of setting up an HTPC for them that they interface with directly. I personally think the full Windows operating system has wait too much baggage. Too much can go wrong, especially as you make more changes. Maybe I just have bad luck (for instance, anytime I ever installed Shark codecs, live TV would just completely break, forcing me to reinstall Windows), but that's my experience. And don't get me started on Catalyst...
Anyways, I also have things set up this way at home, and for personal media I've starting using OpenELEC. I have found the OpenELEC + WMC extender combination excellent, and I will never use a Windows based HTPC again.