From the description, it really sounds like it must be a software thing. Hardware faults don't result in perfectly clean shutdowns.

Note that PVRs don't have to be powered up all of the time to do their thing -- our MythTV box is only powered up when actually recording or when we're viewing recordings. The rest of the time it completely shuts down, using the real-time clock "alarm" feature to power on again automatically for the next recording.

It is possible, I suppose, that SageTV might also have that feature, and somehow it got activated on your box. Or more likely, some WinUpdate has enabled a different power profile than what was in use before, and the system now shuts down after an extended period of "inactivity".

Cheers