After making changes to your registry, it's possible that your computer doesn't boot up anymore.
When this happens the user starts to panic.
The normal procedure is reinstalling the operating system.
However Registry Explorer offers a better solution.
To fix the problem, you will have to remember which changes you made that cause the boot failure or have a backup file of the key you changed.
The first step is to mount the hard disk containing the broken registry in another computer that can still boot up.
It is VERY IMPORTANT that this computer runs the same operating system as the broken computer. Put an XP disk only in an XP computer etc.
Make sure you have Registry Explorer installed on this second computer.
Open Registry Explorer and go to the key HKEY_LOCAL_MACHINE.
Now right-click on HKEY_LOCAL_MACHINE and select "Load Hive".
Select the file that is broken. This is usually the file system.dat in the windows directory. This depends on the operating system.
You can now edit the entries in the broken file. Revert the changes you made before.
If you have a backup file of the damaged key, restore this backup.
When finished, unload the hive. Put the hard disk back and try to boot again.