it's a mind trick
As in Jedi mind trick???? LOL

Anyway, i think he wont really do much unless you really pissed him off. hopefully you got the post that he made threatening to hack you. That will come in handy when something happens. or if you just want to make sure it doesnt by going to his ISP.

Like most of the guys said here, AV, updates, firewall, IDS, anti trojan. thsoe would be the things to do. And disable anything that is not necessary. Good passwords also help. also monitor your box to make sure nothing funny is going on.