I can't explain the extreme low number for Vista itself. Of all Windows users on my site in the past 3 months, about 20% still used Vista, versus 28% Windows 7.
Maybe the data is actually even more garbled than I thought, because looking at the numbers, I am beginning to suspect that Windows 2008 (18% in Sean's APUG stats) is actually Windows 7. Maybe Vista Longhorn than represents a late Vista version, instead of Windows 7. With 11%, that wouldn't be too strange in the light of the statistics of my own site.
Windows 2003 is at least a server version of Windows. Used in big organizations / companies to run networks.
Windows ME is a version of Windows that came after Windows 98, but never grew to much popularity, as Microsoft released Windows XP just afterwards. In fact, I think most users switched from Windows 95 straight to XP, like most are now going from XP to 7 while skipping Vista.