Sniffer on my network

[Computer Vegetable <CompuVeg () COLUMBUS RR COM>]

At my office I've recently installed a network monitoring package called
LanGuard.  One of the things this tool does is find network sniffers on your
network.  I didn't expect to see any, but as it turns out one of our
workstations is showing up as a sniffer.

I am unable to find any processes running on the machine with unidentifiable
sources.  I'm also unable to find any known Trojans or other viruses on that
machine.  The only odd thing that I have found is that anytime a network
cable is plugged into the workstation in question, the address 13.10.15.10
shows up IMMEDIATELY in the ARP.

Has anyone seen anything like this?  ARIN says the address is owned by Xerox
PARC, who's admin says that IP is theirs, but not currently in use.

Thanks