Umm if you know his/her IP. you can try the following:

open up a command window (start->run->"command") and ping him (just to **** him off )

Then, if you are running an NT-based windows os (NT/2k/xp) you can type "Net send 12.239.146.24 who are you and what are you doing to my pc?" this will pop up a dialogue on their PC that will say a message (in this case "who are you and what are you doing to my pc?") NOTE: this will only work if they are also running NT/2k/XP.

Apart from that, if your firewall catches it, you ahve nothing to worry about