How to handle illegal termination of the program?

I am writing a program for authenticating the logged in users.
When ever a user logsin i am storing the rights,session key,login timestamp and many other details in a xml in the local C drive.
When the user logsout i am deleting that particular node from the xml.
But in case of illegal termination of the program,that particular session details of the terminated user still remains in the xml.
Can some one help me out how to proceed with this problem.