Thanks for the thoughts, Avtrician but as mentioned in my original post, wakeup happens every few minutes as opposed to the same time (once) every day. To be precise, almost exactly every five minutes to within a fraction of a second according to the log files.
Also per my original post, even with the the Ethernet cable removed the PC still wakes up after a few minutes asleep. There is no WLAN or other means of network connection with the outside world. Magic packets or anything else network-related is therefore ruled out on top of the fact that everything to do with this is disabled anyway in the BIOS and in W7.
Until recently, sleep and wakeup behaved perfectly; sleep after 30 minutes of inactivity (which is still the case) and wakeup either for Windows Update at 3:00am or Media Center TV recording or guide download. I have not changed any settings or installed any new software in a long time - so the problem appears either to be related to an update (Windows or application software) or the PC is infected with something that I can't find; I have done a lot of digging, including as far as root kit detection.