What percent of Windows machines have XP , Vista , Windows 7, 8 and 8.1 installed on them? Approximately.
The absolutely up-to-date figures are available here - http://www.netmarketshare.com/operating-system-market-share.aspx?qprid=10&qpcustomd=0
They show that Windows XP has actually gained slightly in recent months, which is scary.
Windows (all versions) 90.63%
Windows 7 47.52%
Windows XP 28.98%
Windows 8 6.89%
Windows Vista 3.61%
Windows 8.1 3.6%
Windows 2000 0.03%
Note that this is a percentage from all computers, so that if you want to know the percentage of a particular version of out of all Windows you'll need to multiply each figure by approximately 1.1.