Several peple here pointed out that blocking IP adress is not best solution.

I agree with all of them. Also there is one thing that is not mentioned. If someone has hacked your box than it is only matter of time until someone else do the same thing.

Discovering IP address is relatively simple if you know what to look for, but there is so many posibilities. I think that easiest way is to read firewall log (if you have firewall installed).

But if you want to secure your box for longer period, then you must patch the holle, so that all IPs are blocked when they try to brake in. (I mean that access to port/troyan/whatever is blocked, while you can still use online gaming, browsing etc.)

I won't repeat what others already said about installing firewall, patching or even formating HDD (nihil: this is the best thing to do, but most people are scared of it when they realise how much time it takes to put everything back. I personaly use it as last resort)

BTW: I am eager to hear how did you discover the attacker, and what did he do to your box?