How do you mean you want to delete the account? Is it so that other people who use your computer cant use it?
If it is, when you open msn messenger there should be an option to change the account before you log on. You can set it to not remember your password here if you like. If its set to logon automatically youll need to log off - not close msn to do this.
Also the .Net passport can be accessed at click here - you could permenently delete your account here, but this wont be just on your computer.