C#.net using active directory

In an existing C#.net 2008 windows form application, I want to add some security
features. Thus, I have the following questions:

1. What are the options available for adding security to this C#.net 
application and what is the preferred method?

2. I have heard about using the active directory in C#.net applications. Thus,
can you suggest urls (links) that I can use as a reference?