IE 9 Won't be Supported by XP or Vista SP1
Microsoft announced today that they were working on Internet Explorer 9, and that it would not support Windows XP or Vista SP1.
Customers that tried to deploy IE9 Platform Preview on Vista SP1, XP SP3, or older releases of Windows got the following message from the installer: “Windows Internet Explorer Platform Preview doesn’t support any operating systems earlier than Windows Vista SP2.” This aspect is evident in the system requirements outlined by Microsoft.
IE9 Has No Love for XP SP3 or Vista SP1, and No Support - Not as Platform Preview, and not in the future - Softpedia