In desktops Windows probably hold around 95%, yes. In servers its nowhere near - Windows is around 50% and the rest is made up of Linux, UNIX and a very small numbers of Apple servers. Also keep in mind that almost all these figures are based on sales, not volume and since Linux is free (and often downloaded free off the net) a lot of Linux installs aren't counted in these statistics.
Linux counter tries to provide an estimate of the number of Linux computers in use: http://counter.li.org/
but it is a very rough estimate and probably not statistically valid.