I know how , but it's hard to explain... actually you need to boot on a disk ( Bart_pe or Linux STD ) and then copy the ''SAM'' file and the ''SYSTEM'' file in the path C:\WINDOWS\system32\config. you can't simply copy them from windows because it is being used by the system.
Once you extracted those two files you want to ''BRUTEFORCE'' them using L0phtCrack.LC5 (app). It may take up to two days depending of the complexity of the password. Once you did that you will see all the admin and their password.
( all this is if you want to enter into a computer using windows xp that use a password that you dont have )
oh and if the computer don't boot automaticly on the CD, AND if the bios has a password, then your fucked.
There you go...