No I wasn't using Win 7 On the last Q9450, and I've only experienced this with my new system, i7920 + win7.
The screen shot above shows my second render and the parked cores seem random, but in the earlier render the parked cored were definitely the "hyper threads", cores 0, 2, 4 and 6 were at 100%, cores 1, 3, 5 and 7 were at 0%. My power saving is set to maximum performance and I cant see any changes in the advance setting that seem relevant. I've done the same scene with Ray Trace objects enabled and disabled, with the same results. This is actually the 720p scene you recommended I made last year. A whole series of smaller test renders I mad earlier this evening did not exhibit this behaviour.
Thanks
Richard