I believe the data is global so the numbers are probably skewed by China and other sources as well. FWIW, I sell a few add-ins for Excel, targeted at B2B users. I get roughly 60k uniques/month, and my Windows visitor stats are:

    7     - 61%
    10    - 28%
    8     -  8%
    XP    -  1%
    Vista - <1%