I want to restrict 10 drives in Active Directory (Windows 2003). How can I add Registry Key in AD to do that?

You can use Group Policy Objects to restrict access to any combination of the A, B, C and D drive letters, or to restrict access to all drive letters. If you need more granular control than that, you can roll up a custom .ADM template to restrict access to the particular drive letter that you need. See the following KB article for more details.