From the IEBlog:

 

This morning we released our first public release candidate, IE7 RC1, for Windows XP. You can download it at http://www.microsoft.com/ie.

The RC1 build includes improvements in performance, stability, security, and application compatibility.  You may not notice many visible changes from the Beta 3 release; all we did was listen to your feedback, fix bugs that you reported, and make final adjustments to our CSS support. I do want to call attention to two changes in particular.