// MessageId: ERROR_NETWORK_UNREACHABLE
//
// MessageText:
//
// The network location cannot be reached. For information about network troubleshooting, see Windows Help.
//
#define ERROR_NETWORK_UNREACHABLE 1231L
The 99% case is that the machine doesn't have any routes which lead anywhere, which results from the Windows stack not having loaded any network drivers, because your Windows PE image doesn't have the right network drivers for the client hardware.
So, check what network hardware is present in the client machines and that the Windows PE environment you create to boot the clients has a suitable driver for that network hardware (and if as is likely the case it doesn't, add a Vista driver for the NIC to Windows PE and create a fresh boot environment containing the driver).