Are your booting to WinPE or PCDOS?
If the new DELLS have different NICs to those you have used previously, are you using the correct NIC drivers to allow the machines to connect to the network. If using WinPE, you need to add VISTA 32 bit drivers for your NICS to WinPE as the version of WinPE used by GSS 2.5 is based on the Vista kernel.
Please use the search engine to look for past postings on any techniques you are unsure of, as driver addition is a frequent requirement when dealing with new hardware.