Cheerio,
Is it really not possible to set a power-on password? Having a BIOS admin password is not the same thing.
Your teenagers must be quite savvy if XP passwords are of no use! Have you set up their accounts as administrators?
It is not THAT easy to break out of a plain USER account...
Start by disabling fast user switching...
You may also want to investigate the Group Policy settings which can proscribe activities for some or all users. Best to go to the MS website and search for Group Policy.
Also, with most hardware firewalls you can block access to specific internet sites - e.g. hacking, cracking, etc. if that is where they are getting their information.
SD