His machine is blocking access to your server. Whether intentional or not is up to you to decide.

Yeah I realised this :p It's been blocked since it was created, I've not been able to access it since it was created (the server) - so I don't think it's intentional. What ways are there of blocking IP addresses in Windows XP Pro?
Things I've tried:
Disabling IPSec
Allowing an exception in Windows Firewall (even though I don't use it).
I don't think it's program-related, as when his user is logged off it still blocks access, but when you log off all that is running is the network connection isn't it? Which means it might be network card settings blocking it?

No, it's a setting on his machine, either in the windows firewall, or other firewall or virus protection.

The answer is to get a router and be done... if he won't do that, wait until your 18 and get your own house... lol