If you have IE7 installed on an XP SP-2 machine, are thinking about installing SP-3, and think there is any way you might want to go back to IE6 read this: IEBlog : IE and Windows XP Service Pack 3.

The point is if you won't be able to uninstall IE7. If you think you may need to revert to IE6, uninstall IE7, install SP-3, re-install IE7.