C# and windows identity property

I would like to use the windows identity property to determine who is logged into an existing C#.net desktop 2010 application. Once I determine what user is logged in, I would like to determine what files they should have access to based upon values in the app.config file. I can not use values in a database since the application uses access 2007 database files and each user has there own database files.

Thus can you show me the code on how to accomplish my goal?

If this is not possible, would you tell me what other situation would solve my problem and show me the code on how to solve this issue?