In Need Of Some Admin. User Computer Access Help:

I have a daughter-in-law that I let use my internet connection with
her computer to save her some money.
I have a problem with her downloading files she shouldn't be doing.
I've ask her not to but she still does. I can just disconnect her
completely but I don't want to cause hard feelings.
I wanted to go into her computer tonight and look to see if she had
any illegal files on it and if she did I would block her access from
that site, as I have done with Limewire. I have blocked the ports and
that site in my router.
Now is where the problem is, when I tried to access her computer she
has setup a "Guest" login and password and I need to be able to get
around this or I'll have to deny her complete access to the net if I
can't figure this out.
Is there any way in XP Pro and Vista Ultimate to get into her computer
to make sure nothing illegally is going on or is there some software
that I can get to monitor her activities?
When it comes to setting up Security, I'm not the best with the
systems.
Is it possible I will have to setup my main computer as a server to
prevent this from happening?
Many Thanks
TomP