How to create a .Net application that works as a standalone application or as a pop up within another application without duplicating source code

I already created a stand alone application in .NET and I have use for it as a standalone application.  However, I also have use for this application as a popup within a bigger application.  Of course, I don't want to just copy the code from the smaller app into the larger one, because of duplication of source.

For example, the smaller application is a contact database.  Everyone in my office uses it to manage our contacts.  However, I now have a bigger business wide application that requires links to the contacts database.  I'd like for the business app to be able to pop up the contacts app in order to link records but I don't want to make someone boot the whole business application just to lookup a contact.

Any ideas?