Bah. I doubt it. Even if AOL IM does have security vulnerabilities, your firewalls will get up to block that. and then you have an IP you can report if it does block it. Likewise, ZoneAlarm also has exploits, but it is hard (near impossible) to first get through AOL IM, then through ZoneAlarm, then through WinXP Firewall, and then exploit the FEW (I know they are there) exploits in Mozilla Firefox. Really, it would take about a whole day of high-level hacking just to cause your Mozilla to close. After which, you just open it again.
So I say, he's just lying. Either that, or there's some guy that likes spending a day in coding just to cause Mozilla to quit. And I seriously doubt the second one.
If you get an IP from it all, I know you can pass that onto the ISP. Then that'll be you "hacking skills" to turn off his internet. That'll work. The ISP will just send him a letter, prolly tick off his parents to no end.
But just breathe. No one is likely to get into your stuff. It would take me a year to learn it all, and then all the programs would have changed 7 times by then. If nothing else, get a router. Those are very secure. You add that to your system, he'll be in a wheelchair by the time he breaks into your system.
I've had these hacker threats myself too. It was just I guy who knew how to program an IRC bot. And he thought he could hack me by showing his skills and sending a spoofed e-mail that had my name as the sender and recipient. I downloaded the same prog and spoofed him back, including a traceroute to his comp, addy and all. He never bothered me again. I love working for an ISP.