How do I get AD Users and Computers installed again?

Hi Rod Falanga 

The reg key change they've asked you to make is to get the computer in question to update from Windows Update (like consumers) rather than a centrally managed WSUS update server - not sure why they would want to do this as it sounds from your description than this is a corporate machine, and afaik where you get your updates from doesn't have any bearing on the availability of RSAT. If you try to install RSAT AD DS from your optional features, what happens? Does it show an error message?

The registry will have a Microsoft key already - it possible that the particular account you are using to open up reg editor does not have permissions to view it for some reason.