I use the wonderful Analog for web stats. Unfortunately it hasn’t been updated for a while (2004 in fact).
However I noticed that some kind soles on the mailing list have updated analog so that it includes Vista in the OS report.
I’ve put a copy here, just download the zip (Windows), extract and replace the original analog.exe.
See this mailing list message (and the thread that follows) for more information and credit to those who produced this new version.