Works on most all windows machine( MAY be XP only), this can be disabled in BIOS, however. (So if your computer is a floor model or you have a parent who works in any kind of computer career this is probably disabled.)
Step 1: Windows login Screen
(The screen that appears prompting you to log in)
Now, press control alt delete twice in quick succession, opening a default login interface (Basically, its uglier and more complicated
)
Step 2: Logging in
Type Administrator under username and leave password blank. If it was successful you are now on a service account (or ROOT ADMIN) and can change any settings/passwords/etc. *or just play on the internet =/*
Congratulations, now mommy can't take away your computer.