XP-Hidden Window's XP Admin
This is an easy one if your parent blocked your account, or if you want to hack someone elses account on your computer (or at school)
Log out of all users then tap CTRL+ALT+DELETE 2 times
A little box will pop up
Type "administrator"(or "admin" (without the " ")) in the user: box
Then hit enter, it will work unless someone else knows about this in your system or they changed the password.
An alternative way to get into admin privilaged users is if you have system write privilages, then go into your windows system32 folder and find cmd (or cmd.exe) copy and paste it onto your desktop, then rename the one on your desktop sethc and drop it back into your system 32 folder. Now log out and hit SHIFT 5 times, the cmd box will pop up. In this box type "net user EXAMPLE *
A new line will ask you to write a new password and confirm. Your done.