Installing Small Business Server

I have purchased a Small Business Server (not loaded yet) from Dell. There are two companies currently sharing one NT Domain but the companies will be splitting in the future. I want to install the Small Business Server so that the company that is going to be moving can take it with them and have most of their files and programs.
My issue is:
Small Business Server requires Active Directory. If I load Active Directory I do not want it in any way mess up the NT Domain. The company that will bemoving needs access to the current infrastructure but the company that is staying does not need access to the SBS.
My thoughts:
Install Active Directory as a root domain controller and then setup the Small Business Server.
Looking for your thoughts and suggestions…….