You can try booting up in Safe mode by repeatedly tapping the F8 key when you turn on the computer
then when you get into windows go to: start-run- then type: control userpasswords2
select the user password that you need to edit then hit "reset password"
it'll ask you for the new password twice enter your new password reboot and that should work
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If it doesn't work try this:
click start then run then cmd
this will bring up command prompt
type cd/
this should get rid of the documents and settings part of the line and only show C:/>
you can then type cls to only show C:/>
now type "net user administrator xxxxx"
(without quotes)(the x's are where you can put a password)
type a password where the x's are and this should change the admin password