My temps for my cpu are constantly 2-3 degrees Celsius warmer while running vista, however I have a new AV which I believe is normally or constantly scanning. Avast, unless I am just wrong there I don't know.
Well, perhaps it's doing stuff in the background? Indexing?
I haven't bothered to compare my CPU temperatures between different OS's, but the fact that yours are different doesn't shock me. All OS's are different, and some use more system resources to run (Vista being the one in question).