IMV this is an impossible task to achieve while keeping the PC useful for anything.
I tried it (have 2 boys) a couple of years ago and eventually gave up. Net Nanny is a piece of cr*p which has various bugs in the way it applies its rules, and there was no way to make it run its blacklist while allowing access to a model aircraft shop ("model" on the title").
It takes the average boy minutes to discover that entering certain keywords (e.g. "oral", to start with the really mild mainstream stuff) into Google and selecting Images will bypass anything you have set up; the hardest core stuff appearing immediately. No need to go to pay websites (most really dodgy stuff has to be paid for, I am informed
); this simple measure reveals enough to keep him going for months.....
The much bigger risk is the kid going to some website which trashes the PC. This is a
real problem. One must
- apply all the micro$oft patches
- have a NAT router whose config is password protected and he can't get in (to open any ports) - do not allow any P2P software
- run an updated antivirus program (Kaspersky Personal pro is my choice but the others usually work too)
- not have the machine connected to a network with others on it, and if this cannot be avoided then at least run it under a login which a) is not an Administrator and b) the login/password is not the same as the rest of the network.
- not use the machine for anything seriously important
- if you want to really p*ss him off, inhibit file downloads in Explorer - though this is easy enough to circumvent.
The above said, a kid will always trash a PC eventually (to the point where it needs a reformat from the shop recovery CDs), if only by loading into it every CD that comes on the cover of every computer mag. My son has trashed every PC within 3 months, totally, and probably none of it was through a virus.
Kids are reckless, and very very fast; much faster than you can watch them. They will click on every "OK" box that pops up, so fast you will barely see it. They don't care what the dialog box said. The answer is always "OK".
If doing something known dodgy, they will hover their fingers over the alt-F4 keys and press them the instant you walk up to them, etc....
The joys of being a parent