Run a utility to check for worms. They may get through your virus software and for the past month or so they seem to be hitting a lot of routers, and assorted hardware. It doesn't hurt the equipment but it may cause them to reboot periodicaly and it definitely increases your traffic on the network. Stinger.exe is a Network Associates (mcaffee) scanner that will search a machine only for worms... I think they are up to about 35 or so varients. A packet sniffer is a good tool.. but expensive for a "one-time" solution, which hopefully it is.
Get XPSP2 (service pack 2) when it comes out later this month (possibly) and install it on the XP machines. We've been seeing a lot of traffic lately on our XP machines on port 135 which I think is related to the remote features.
You could install BorderManager on the Novell server to act as a firewall and then see the activity for yourself. But that again is a more involved installation.