I have an optiplex 980 system with a NIC from intel model 82578DM. From a quick search i saw that that this message might be because an unsupported NIC.
I've downloaded the ESXi Dell Customable CD.
does anyone have any idea regarding this issue?
It seems your system and nic are not listed in the Compatibility Guide as a ESXi 4 supported hardware:
The ESXi 4 has not a driver embeded for your NIC.