Will WPF run on XP and Vista?
WPF is part of the .NET framework. As long as you have the right framework installed it will work. WPF was introduced with .NET 3.0. If you plan to use it, I recommend going for .NET 4.0 as it has matured with each version.
More info: http://en.wikipedia.org/wiki/Windows_Presentation_Foundation
WinXP was not shipped with any .NET Framework included, it has to be installed separately. Vista included 3.0. Windows 7 included 3.5. I think that it is safe to assume that 99% of all Windows computers out there have a recent .NET framework version installed since so many apps require it.
XP support is not recommended, i just had the case of the Window.Loaded event being fired too early ...
As long as the correct version of .net is installed (3 or 4) on the machine then yes you will be able to run applications.
.Net should come installed on the users machine out of the box. Though it may require some updates being installed in order for it to be at the latest version.