windows password breakers

i have a truble maker that keeps going into computers
on my home network and changing passwords or using
programs to put passwords on files… i was wondering if
1 there was a simple way of stoping him and
2 f anyone knew a program that can get buy a
password on a computer… can get into the computer
but when i go to the file it asks me for the password…
wch i never set… i have been able to get arpund the
passwords by going into sub files then hiting the go up
one button but i would like to figure out the password so
that i can change it
i also need a software that tells me the IP address of all
the computers that contact my windows XP computer
i would like to spend as little money on this progect as
posable so links to free software that would do this
would be muchly apritated