This is either due to a lack of network drivers, or due to the drive mapping command, net use, executing too early in the boot sequence.

First, you must verify that you have networking capabilities in the boot environment. To do this, boot with your existing Windows PE boot disk , then choose Quit to exit Ghost. At the command prompt, type the following, then click Enter:

ipconfig/all

If an IP address is present, and is not 169.xxx.xxx.xxx, then you have networking capabilities. If this is not the case, you will need to add the appropriate network driver according to these instructions:

How to add drivers to DeployAnywhere or WinPE.

http://service1.symantec.com/support/on-technology.nsf/docid/2008050916132860

If you have networking capabilities but still cannot map a drive, it is likely that the net use command is executing too quickly. To resolve this, recreate your boot disk as normal, but with the following exception on the page titled "Symantec Ghost Boot Wizard - Review" page in the Ghost Boot Wizard:

1. Click the Start Editing button in the lower right-hand corner of the window.

2. Click on the line just above the line starting with "net use".

3. Type the following:

ping 127.0.0.1

4. Click Stop Editing.

5. Click Next to complete the wizard and create the boot media.

This should work to resolve it if the issue is with timing.