This reiterates a few previous posts, but here are some comments in a nutshell:
- The T400 won't come close to running WoW, even if it has the premium discrete graphics that were offered at the time it came out. Integrated graphics? WoW won't even load. Build or buy a cheap desktop from a couple of years ago with a decent $50 graphics card and you'll be much better off.
- Windows 7 + microsoft security essentials will keep you secure. Then you need to teach your son the same basic internet security you need on any net-connected device -- don't fall for phishing or fake download schemes. There's no reason to buy third party AV.
- If you're buying a new OS license, you probably should just get Windows 8. It's less resource intensive than Windows 7 in every way, so if your son is trying to game on legacy hardware, that will help. Additionally it has full antimalware built in, along with a raft of improved security features. And finally, with the special offers these days, it's probably cheaper (if it's not, you're probably not getting a legal Win7 license).
- I'm of the opinion you give your son full admin to the computer, and if he breaks it, well, he learns how to fix it. Restricting him to a limited user account just means it's harder for him to learn. You should take other steps to protect your network in case he busts his PC, but that's awfully hard to do with Win7/8 and basic AV software.