Workgroup is the default domain/network that Windows selects upon initial setup of the OS. You might want to see if any of the machines are currently part of Workgroup. I've seen 98 (all versions) machines continue to show the Workgroup domain despite the fact that no machines are connected to it.
As for the other mystery networks, it might be possible that a machine was setup to be on a different network before joining your network.
A few questions:
What operating systems are you running on your machines?
How many machines are on your network?
Were each of them fresh installs of Windows when they were hooked up to your network?
What operating systems are you running?
What networking equipment are you using?
Edited by Myke (01/30/07 11:03 PM)
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I swear, troubleshooting is a science: the best discoveries are always on accident...