not sure if cybersitter or netnanny might be what you are looking for.
they are NOT free but do have a trial version which runs for i think 30 days. might just shock the kids into behaving if they think they are gonna lose access.
other thing to do is to stop access to the internet on the kids accounts. create a new user with internet access but dont reveal the password, log on for them and log off when you go to bed or whenever you think they have been on enough. copy and paste the downloads, let them then login to their own accounts and play the music etc.
not ideal i know but i think you are pretty limited on a XP Home version.
others on the forum will know better than me on this one.