C#.net accessing files

In a C#.net 2010 desktop application, there is a dropdown list where the user selects who they are. From this list, the application knows what files are unique to the user.
 In the past there was only one user. Now there is going to be 4 to 8 different users. The dropdown list is used to point to where the files are located for each user. This occurs since the user selects their individual names from the dropdown list and all the files uniquely start with the name of the person. In other words if the name Joe is selected then all the file names are Joe_file1,Joe_file2, etc.
 The users are concerned that they will accidently update each others files. The users do not want to this to occur. This solution was determined by the previous programmer so each person could backup each other.

Thus I am trying to determine what a good solution would be.

Here is what I am thinking a good solution should be:

1. Change the application to determine what windows logon the person is using. Possibly the windows identity property could be used. This way the application will determine what files the user has access to.
 2. Have another windows user created that basically has admin rights. This admin user logon would have access to all the users files. Basically when a person signs on with the windows admin user account, they would have to know what files they are trying to access.