Well my description was not clear enough.
The machine has one network card. It is normally running at home, on a LAN without DHCP. It is then configured manually, with a fixed IP address (for the machine, the gateway, the DNSes). I took the machine with me at the office for a few days, where there is a DHCP server running. In order not to change and loose the existing network setup (not a big deal, but it has an unusual netmask and IP numbering), I wanted to create an new "network" in Network and Sharing Center, which to apply DHCP to. This is what I am unable to do. The option to create a new "place" does not exist anymore on that machine. Do you see what I mean ? I suspect that I switched off in the past some Windows features related to the network, but do not remember which one.
I hope that description of the context is better ? Thanks for any help.