You should easily be able to do a System Restore - unless you specifically turned it off. The "System Restore" point saves your Windows state (including the entire registry). Just click the "Start" button and type "system restore", it will pop up, hit "Enter" and restore to the point it automatically made before it installed Internet Explorer 9. Your PC will take a while, and then reboot. Make sure everything is working right for a day or so, then download the installer from the Microsoft website.
system restore i will do, but don't u think that he should boot into safe mode to do, because i think he cannot boot into windows right? so if he just press F8 at boot then get into safe mode to do that, i think its much easier to do that in this instance if he does not have a recovery disk.