Administrator and guest are built in accounts that cannot be deleted, they can be renamed though, why I hear you ask? Well everyone knows that the top of the NT tree is the administrator: the account that has all the power so that is the account that hackers will try to use. Microsoft suggest you rename the administrator's account as something bland then create another account call "Administrator" with no privaleges. The guest account is usally disabled by default and has no privaleges and is designed for network administrators to have an account for, say, temp workers. The "all users" is very useful especially the desktop setting: If you have a shortcut to a work Access account and all users (even ones that have not yet created an account on the machine) need to have it as a desktop icon just paste it onto the all users desktop folder. Once again, this cannot be deleted. The owner is the first account created when you first register the machine and is equivalent to an administrator. For some strange reason you cannot say that your name is administrator and you log on with the administrator's account but instead you have to create another admin account. Not the best way to secure your computer. I log on as the administrator then delete the owener account.I guess the default user is a template. If you look at the resources they take up, it is very small in this world of 120 GB hard drives. The short answer is that most of them are neccessary, so don't go deleting them!