Hi, I'm a computer analyst so I thought I should respond.
The thing about computers is that if you're knowledgable enough, you can pretty much get around any restrictions someone else might put in place. So if the son knows more about computers than the mom, there's not too much that can be done from the technology side. But here's some tips:
1) In Windows, open the Control Panel and go to "Add/Remove Programs". Kazaa is only one application used to access peer-to-peer file sharing networks; there are many others. Try removing the applications from this interface.
2) Your friend should install a firewall. You can do this either with software or a physical hardware device. The firewall should be set to block port 6346, which is what Kazaa and other peer-to-peer file sharing programs use. You can selectively choose which ports to block, leaving ports open that are used for email, web browsing, etc. and blocking all others. Not only will this prevent peer-to-peer file sharing, but also protect her computer from malicious Interent attacks.
3) With Windows XP, you friend can create an administrative user account for herself and a limited user account for her son. Her son will be able to use all the installed applications but not install applications on his own.
4) Don't waste money on Internet filtering software. It just doesn't work. It blocks plenty of legitimate sites and still lets plenty of porn go through.
I'm sorry to say that if she wants to have an Internet connection, there is no way to absolutely prevent her son from downloading porn if he chooses. I would be interested to hear how parents of teen deal with this. Do you check the Internet cache after your kids browse? Keep the Internet on computers only in common living areas? Or just talk to htem and hope they make good choices?
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