Still couldn't fix Freddie's machine. Earlier today I took the wifi card out and stuck in a wifi dongle instead, and it reports this connection firewalled as well. It's strange because it could see the router.
Anyway, it's happily running Edubuntu instead of Windows now and apart from needing the dongle because there were no Linux Broadcom drivers all is well. Installing Ubuntu was easier than f*ck about trying to sort it out and keep Windows...
Freddie likes Tuxpaint, so double bonus.