You don't need to keep 10GB free. It is a good idea to keep at least a few GB, and the more the better (for performance, disk-read-write-speed), and Windows uses some space for virtual memory. E.g. can't you store the anime on secondary storage, like another hardrive or DVDs.
A lot of IT people don't like Norton. There is no reason not to, it was on the IBM desktop when I worked there. It does hog system resources, though, and can clobber some games sometimes. The AV doesn't stop spyware.
He sounds like he had a point, but was also taking the pee. Ad-Aware and Spybot are not dangerous. NwN does connect to the internet if you are playing a Premium module. Firefox can connect automatically to update themes and plug-ins, but this is just the same as Windows. Windows Defender is unproven and still in beta, and (judging by the way Microsoft permitted certain paid-up advertisers' adverts unfettered access to people's PCs) I wouldn't be rushing to install it. I currently run Norton Internet Security (software firewall) instead of the default one included with Windows.