Is there a way to rebuild the tcp/ip stack in a MAC if this is the only solution available?
MAC is the address of your ethernet device.
Mac OS is the operating system. :P
And yes, simply go into system preferences and toss the preference file.
I think its here -> HD>Library>Preferences>System Configuration>preferences.plist
Although I doubt that's your problem, I think the Cisco client is the issue. Don't know if it will work, but best bet is to reinstall the client.