That's interesting. I was aware of some of the security APIs used in IE9, but in those cases I always thought that it was still essentially a business decision not to provide an XP-compatible version. IE9 is supported on Vista as well as 7 though, and it seems that they had to backport some APIs to allow this (see KB2117917). I don't know enough about the underlying code to know how hard it would have been to do that for XP, but I can definitely see that it could be a significant effort.