The best solution is to have your brother undo the changes he made. Only he would know if he changed anything else in the registry. If he can't (or won't) undo the changes, hit him a few more times with that 2x4.
I have two possible things you can try but absolutely no guarantee either one will work. Before trying either one make a new restore point (or better yet, a system image) just in case things get worse.
First possibility: Last Known Good Configuration. LKGC has backups of the registry hives. But if you've rebooted the computer since your brother made the registry changes, then those registry backups will have also changed. But if you want to try it, you need to access Advanced Boot Options menu. Advanced Boot Options
Second possibility: A repair install. This will try to fix your currently installed Windows 7 by using default configurations while preserving your user accounts, data, programs, system drivers, etc. You will need a Windows 7 installation disk that is the same edition of Windows 7 that you have installed. Repair Install
Perhaps some other Forum members will jump in with better solutions.