The first release candidate for Windows XP SP 2 is available here. The changes are described here. We've made a lot of major security related changes for SP 2. We can use all of the feedback we can get before it's rolled out to everyone. So, if you're up for testing out a version of the OS that might still have a few bugs left (as opposed to the final release which will have 0 bugs :-), please give it a try. If you are a developer, you should definitely try your applications out on it to make sure there aren't any issues.