Thanks a lot guys..... Some how i have recovered all my files

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Now i have another problem....
I have 3 user accounts(Administrator, XXX, YYY), this 3 have administrative privilege and it is password protected. Now when i start my computer it automatically enters into administrator account without asking for password...
I want sollution for following problems :
1. I want all the users to enter the password and then to enter into a profile..
2. I wanna make XXX or YYY to be the default user name in the login field...
3. Is there any other ways that the users can enter without knowing the password??? If so What to do to prevent this???
Thanks in Advance.....