As of right now there's no way to run IE9 on a Windows XP machine. For me that's not a problem because I develop all of my websites on my Windows 7 machine. If I need to fall back to windows XP with IE6, IR7 or IE8 I have a laptop with all of them installed for testing. I don't need IE9 on that laptop.
I have been using IE9 off and on since it's been in Alpha and I have to say that I'm impressed with the Internet Explorer teams attention to Web Standards as well as their support for HTML5 and CSS3.
I am more impressed that the team has left XP users by the wayside this time in hopes of advancing the web in general. Thank you Microsoft! If other browsers would do the same the web could move past an operating system that's over 10 years old, filled with security holes and lacking support.
Of course they have a more important goal here, to control the web. The company that has the largest browser share will control the flow of information. Microsoft is definitely interested in that, who wouldn't be?
If you are using Windows 7 and don't know which version of Internet Explorer you are using you should definitely check out IE9.
Good job IE9 team, you rule!