Selectively block access to certain internet programs on Windows XP

[#1]

Situation is:-
One Windows laptop with 4 users (all as administrators)
Two children - one abuses it and uses things such as MSN chat, Facespace, etc, etc - simultaneously doing her homework.
The other does his work and uses some of the above - after he has finished his homework.
Is there any method of blocking the bad - but still allowing the good ?
Is there such a thing as a user-specific software firewall (I don't have a software firewall at the moment)?
Before anyone says anything, all users are set-up as admin because it was "easier" and Windows is so bug-ridden anyway.

[#2]

I was reading about Windows SteadyState the other day which might do what you want. I'm still researching so haven't installed it yet. Perhaps some forum members have?
http://www.microsoft.com/windows/products/winfamily/sharedaccess/seeit/athome.mspx

[#3]

I neglected to say that I have a very strong preference in favour of freeware.
So I think that rules out Microsoft for me, anyway.

[#4]

C:\WINDOWS\system32\drivers\etc
Open the Hosts file with Notepad.
Scroll to the bottom and enter e.g.:
127.0.0.1 www.facebook.com
close and save changes, now you won't be able to access Facebook from your computer at all!
Repeat as necessary.
Your router should be able to block any sites  (if you're using a router).
As you suggest a software firewall should be able to do it too.
Regards,
Neil

[#5]

There are several solutions depending how severe you want to be and if its only at certain times or all the time
You can use built in windows group policies to deny access  to anything like control panel MSN etc.
One of the best, simplest and Freeware is to use a proxy server from
http://www.handcraftedsoftware.org/index.php
Documents here
http://www.handcraftedsoftware.org/index.php?page=5
As you can see there is loads of support and forums to help some of the things it can do are
          Allow access to the internet at certain times
          Allow some users access to some sites
          Allow some users access to a selection of sites at specific times.
          Allow the import of ban lists to ban based on URL or IP address
          Access report: detailing access violations
          Detailed log of web sites accessed
          Summary report of access by user
          Debug report
As you can see it does all you want and the best is that it reports everything so you can then grab the kids and say you went on this site at this time and tried to look at  unsuitable material. You can also set it by time so they can only use MSN after there home work for example

[#6]

Thanks Samuria,
This sounds simple enough for me to use! 
I especially like the access logs features.