iMac networking Native Ethernet

I am helping a friend to setup a network which consists of Win XP machines and an iMac OS 10.4 (?). We are using a Speedstream DSL/Router and Airport. The Airport is being used as a switch at the moment. The Win XP systems work fine across a wired network. The iMac works for a while across a wireless connection. The Airport drops for some reason?? Not sure why. I try connecting the wired network to the iMac and I get message "no cable
connected". This is a hard wired connection and my Win XP laptop works fine using the same physcial connection.

When I check the Network settings on iMac I see that native Ethernet has an IP address of 144.xxx.xxx.xxx and it should be 10.0.1.x. I have reset the DHCP and no change. It is as if the network adapter in the iMac is not functioning?? In the Win world there is a command to reset IP - netsh interface reset ip. Is there something in the Mac world that will drive a reset to the IP interface?

I've tried powering the system up and down no change. I seem to remember that somewhere there is a parameter to set in a file - outside of the system preference windows?

There is no problem with the wired connection itself. I suppose the net adapter within the iMac could be defective?

Other suggestions?

Thanks

Power the router off and back on (I assume that your router is your dhcp server).

Yeah, I've done that may times. I have no problems at all connecting a laptop running Win XP. Connection is good always with Windows.

I believe the network adapter is not communicating out to the router.
Even if I hard code the IP address, it does not work.
The adapter needs to be reset or it may be defective.
I'm sure there was a file that required a small change to tell OS X that
the network adapter was connected. If the adapter could be removed easy
it might help, but that is not possible as it is on the MB.

It may help, if you have the restore CD's and you and you can boot off them and it works fine then it's probably a preference. Maybe

/Library/Preferences/SystemConfiguration/NetworkInterfaces.plist or one of the other ones in there. I ran into a similar problem and ditched /Library/Preferences because I didn't feel like tracking it down.. and it resolved it.

Thanks, I try your suggestion.