Thanks for the suggestions. Quoting "Ziliang Guo" <ziliang@xxxxxxxxxxx>:
Set STARTD_DEBUG = D_FULLDEBUG | D_SECURITY | D_NETWORK in the config file and restart Condor on one of the machines you believe is not reporting. You'll likely also want to increase the max log file size for the startd log. Look inside the log to see what IP address it is reporting to the central manager and whether it actually matches your machine's actual IP.
I have only attempted to add one Windows 7 computer to the pool, but I edited its condor_config STARTD_DEBUG line to be the above and increased the max log size. After restarting, the reported IP address is as expected (matches that reported by ipconfig).
Another possibility is that you have virtual machine software of some type installed that created a virtual NIC that Condor is latching onto.
I don't know of any installed on that computer or any of the other computers.