Here's what you do:
1. Get into a DOS prompt, ok, ow try to trace where the error is coming from at start up. Once you figure out the guys IP, just get in dos again, and upload a virus right up his ass. I've done that many times, it works.
2. Pull out your modem thing dude, if he's getting you through your infared modem thing, just rip it out and get a new one, a 56K modem is like $20.
3. If he harrassed you, geta shotgun, and if he tries it again, blow him through the wall.
I suggest #1, but if that dont work do #2, and if that still dont work, use #3, you should be doing that anyway.
P.S-if you have Nortan, anyone or anything that accesses you computer will be listed in the access menu, see anything irregular, you know that's got to be your guy.