Baffled by security problem

My son runs his internet connection off of a router connected to a cable modem. The other week his apartment mate got a computer, and he was using AOL Instant Messenger. Suddenly he was able to see every keystroke my son was making (relgardless of program) inside IM.

We made sure everything was up to date, ran the requisite security programs (and then some), and when that did not resolve the problem, I did a fresh install of XP and all of his software. Plugged it back in - same problem.

Anyone have any idea of what could be going on here?

Many thanks!

 

Let me ask the obvious question - are they both using wireless keyboards?

Blue

 

Oh my goodness. I never thought of that, and that is exactly what is happening - same keyboards/mice/etc.
Thanks so much!!!!!