There is a problem with your setup. (Nice catch Bruce....)
Unplug the USB cable from the modem to the Win98 PC and then reset the router. After this, start AIM from the other XP PCs.....any luck?
The USB cable acts as a direct serial link (All networks are serial) to the modem, thus interactively negating the network connection to the router. To permenatly fix this problem, connect the Win98 PCs Network card to an open port on the router. Also be sure that the DSL modem is connected to the "Internet" or "WAN" port on the router and the PCs are connected to the numbered ports.
You will no longer be using the USB cable. If you don't have a network card on the Windows 98 machine, then you can either purchase one at Best Buy for like $20, or I can sell you a Linksys Cat5 > USB converter for like $10. These are very handy....
-Mike