Can't get any IP address from DHCP, but unattributed IP adress "already in use"

Hello,

I have a mac mini with a manual IP address (192.168.1.121) which connects fine to the internet through ethernet.

When switching to DHCP instead of manual, no IP address is ever given to me (blank space after IP adress: in the network prefpane)

however, I immediately have an alert popup that says the address 192.168.1.150 is already in use.

I wonder where I could change this 192.168.1.150 address since I do not have this setting at all on this machine on any Network Port, not in Airport and not in ethernet. (I have no other network ports)

How can I change a setting that I didn't make in the first place?

The console reports the problem once every second also, and there is quite a general slowdown and beachball because of this.

I tried to renew DHCP lease but it stays blank too.

I also try to delete ethernet interface, and trash the Network configuration in the general Library, reboot, recreate it but the problem remains.

I also have reinstalled Snow Leopard and applied updates with no better luck.

Thank you very much everyone if you have a clue.