This happens when;
a) DHCP is actually down/broken... or ...
b) The PE disc doesn't have the appropriate network card driver.
Make sure that DHCP is handing out addresses. Try a release/renew on a working test system. If all is good there... make sure that in PXE Configuration, your PE image has the appropriate drivers loaded for the network card. Use the device manager's "Details" tab to get the vendor and device id of your target computer's network adapter and then ensure that it is in the list for the the PE disk.