Application closing problem

Ok this solved the problem of WorkForm closing, and I solved the problem of multiple logging by opening forms as modal, rather than non-modal, so this prevents user to access the Login form as long as Administration or WorkForm is opened. 
Concerning user requests, the problem is that he has no knowledge of programming, so he just gives me new requests when he thinks of some functionality that he forgot about in our previous conversations. It does create some problems for me, but honestly I was prepared for this, so I cope with it as best as I can, and I don't think that will change soon. 
Anyway thank you for your help, I thought that I tried the Close method, obviously I was wrong :D

Yes, I think that what Pravin says will solve the main problem of the application exiting before it should.

Also I hope your user understands that it helps to provide all the requirements before any work is done.

It sounds as if it would help to have a better design for managing all the windows and such. I might be able to help but that would require more time than I have at the moment. Hopefully I can get to that later.

Yes I changed the Program.cs so it states Application.Run(new Form2()); the Form1 is closing by  System.Environment.Exit(-1); (in it's Form1_FormClosing method).
I'll give the forms logical names so it will be easier for you to follow: Form1 is the "WorkForm" since it does the actual work, Form2 is the "Login" form and Form3 is the "Administration" form.

Let me describe the steps that led me to this problem. When user first gave me his request he just described the functionality he needs - so I created a new project (which of course created the Form1 - WorkForm) and made the required functionality. When I presented the application to the user, he had some additional requests that were not specified in the original request - one of them being the login to the application, so that in the final report he can see who was working on the application in a certain time span. I solved these new requests, and I added a new form (Form2 - Login form) that controls the login process (if the user has the administrator access he can chose between WorkForm (the original problem application) and "Administration" (control of user accounts etc) and if he has the "operator" access WorkForm is automatically opened (obviously he shouldn't be able to access the administration)). I managed to make the application do this, and the functionality is more than satisfactory, except for two problem. First, I need a "logout" functionality (I've put it in a menu strip) that will close the WorkForm or Administration, but leave Login so that another user can log on to the application (I managed to "reset" the Login when a user logs on - this way when forms are closed - by both logout and Close button - the user name and password textBoxes are empty and ready to accept new users), but the previously mentioned problem appears here because when WorkForm is closed, it closes the login form as well (Administration doesn't, that part is working fine). The second thing (I really didn't start working on that so I don't know will it be a problem, but if you have any suggestions please share) is this: How can I restrict the user so that while forms WorkForm or Administration are running, he cant use the Login form (obviously two users can't be logged on at the same time).
I hope that I gave you the informations you need, so if you have any ideas please tell me. Thanks

Just to be clear, do you have this line in your Main() method in program.cs:

  Application.Run(new Form1());

and, if so, are you calling Application.Exit() to close the whole application when Form2 closes?