Benjamin Gray, an analyst at Forrester Research says "Vista's biggest competition is not Apple or Novell or Red Hat; it's Microsoft itself, it's XP".
Many enterprises are not willing to move to Windows Vista because of Windows XP stability and wide range of third party applications support. This is forcing Microsoft to extend support for Windows XP operating system. The Service Pack 3 of Windows XP scheduled to release by end of March 2008 is expected to boost performance of XP.
There are many reports by early beta testers about the performance boost in SP3 of XP. The news article Windows XP Speed Boost Coming with SP3 reporting 10% boost in performance while running Office
Running an Office productivity test suite on a preview version of Service Pack 3 for Windows XP, Devil Mountain discovered a 10 percent performance boost over the current version of Windows XP
Another report Windows XP outshines Vista in benchmarking test from CNet News says
Vista, both with and without SP1, performed notably slower than XP with SP3 in the test, taking over 80 seconds to complete the test, compared to the beta SP3-enhanced XP's 35 seconds.
It is quite interesting to see Microsoft products competing with each other.