I had the same issues, I have the correct drivers, however i can't for the life of me remember where i got them. I posted them for ya.
edit: i believe i got the driver here - http://www.broadcom.com/support/ethernet_nic/netxtreme_desktop.php
Dell D600 Network.zip 120.1 K
I used your drivers since they were newer than mine. I still had the problem.
I since then found that I have GX260 that do the same thing. The error message says to check DOC-2158 which states that winpe need a valid driver and how to add drivers to the winpe program.
Well it just so happens that we have some GX260 that do work. And that led me to a different direction.
Found out that one GX260 had 256meg memory and the other that worked had 512.
We increased the on board memory to 512 and it worked on both the GX260 and the D600.
Thanks for your help.