I use Statcounter.com, an excellent free service. It records stats for the most recent 500 visitors, and thus provides a rolling snapshot of Internet user preferences.
MSIE is split between versions 6, 7 and 8. MSIE 6 is the bugbear of webpage designers due to its many faults that require workrounds, but it is now down to 11%.
It now seems safe to regard 1024 as the lowest width that needs to be accommodated. 'Unknown' is surprisingly high, looking elsewhere it seems to be mostly variations on 1280 such as 1280x800.
Windows XP 54%
Windows Vista 28%
And that with new improved Windows 7 almost here is as high as Vista will go. It remains to be seen how long businesses will hang on to XP 'because it works' and remain unconvinced that upgrading would be a financial net gain.