installer does not find a network - but a live system does ... wtf?

[Andi Meier]

a newly installed Ubuntu 10.04 is working - well, almost. there is no network, I have a DHCP server but somehow the system would not receive
any DHCPOFFERs - I don't know why. the DHCP server is up and running, the network is attached, all is well.

odd: when I boot with a live Ubuntu (via USB pen drive), the same machine HAS network access, RECEIVES DHCPOFFERs, CAN access my other
machine etc. so, the network itself is OK, so is the networking hardware.

But the system (when booted from the harddisk drive) is not able to access the network.

what's more: even if I reinstall the system, the installation routine does not find any network and tells me:

  "Network autoconfiguration failed

  Your network is probably not using the DHCP protocol. Alternatively, the DHCP server
  may be slow or some network hardware is not working properly."

WTF??

I do not understand how DHCP functionality could not be working for the installer and the installed system, but works when booted with
Ubuntu live ...?

Andi