Windows

#1 modifing the registry is a last resort method to solving a problem. #2 if you don't know what your doing modifing the registry, you'll cause more problems that you started out with. #3 if you muck up the registry, you'll need to probably reinstall the O/S as undoing what was mucked up is the same as #2 above.

Now you say you need to modify the registry to enter a different key, I'm assuming your referring to a CD Key, that begs the question of why? If you have a W2K CD with the CD Key you aren't gonna need a different CD key. If your trying to do an unattended network install of W2K to a client computer and you need to add in the answer file the CD key, then thats another issue all together.

you may be able to 'recover' your existing windows 2000 install if i read you correctly. see more info at support.microsoft.com. you will need to boot from your installation cd

1st save ur registry entries using export and save it in a safe place.and do wat ever u want and if it does not work and u want to restore it then go to safe mode and import the registry
good luck