Comment Untangle (Score 1) 295
Sorry if someone has already posted this, I didn't see it in any of the popular comments while browsing.
I'm a sysadmin and I use something similar for my networks that's free. It's a Linux based firewall for complete idiots called Untangle. (I don't work for them or contribute to the project.) They have an "app" (also one of the free ones) that runs adblock on everything that passes through the device.
Take any old crappy PC, buy a NIC ($10-$20 investment), burn Untangle to a CD. It's a typical Next->Next->Next->Finish sort of installation, it will overwrite the OS though, so make sure your old crappy pc doesn't have any files you need. There are a bunch of apps, but for this project you just want the free version of their adblocker.
It runs in passthrough mode, so it is super-simple. You can tack on other firewall apps if you feel like it, and the interface is really idiot-proof. Actual Linux nerds are probably pulling their hair out at the waste of resources, but even a crappy old PC isn't going to suffer under this UI.