Well, I ran into a little issue the other day. Loaded speedfan up and there were no HDD temperatures. Thought that was strange, then I saw there was no S.M.A.R.T. monitoring at all! Hmm. So I did a little searching and found zilch. One question at the official Win7 forums asking the same thing with no answers.
So, I get around to getting Folding@Home installed. To do so properly (at least with MPICH), you have to disable UAC altogether. Since it's just my wife and me, there is no reason for it to be on anyway. Loaded up Speedfan...lo and behold, the HDD temps were there.
So, long story short - If you are unable to monitor you HDDs S.M.A.R.T. features, such as temperature, it's because UAC doesn't allow you to. WHY it doesn't allow you to, I haven't the foggiest. There is probably something someone can figure out to have it work with UAC enabled. But the fastest way to get it back is to disable UAC altogether.