Just tracked down and resolved this error, which lead to a recurring management task.

Basically it's caused by a corrupted or outdated PXE image on the Zen server.
Outdated can be taken as meaning 'imaging won't load after the workstation BIOS updates changed stuff' or 'got some ultranew hardware'

The fix: use the latest Novell imaging driver update:
NOVELL: Downloads - ZENworks Imaging Driver Update for February 2009

On subsequent months use the "Superseded by" link which will appear once this image has been updated.

Simply extract the files, backup the originals and replace them in the relevant directory. x:\Program Files\Novell\ZENworks\share\tftp\boot
The files are named initrc, linux and root.
Restart the TFTP service and test the PXE environment.

You may wish to add this as a monthly management task.

This would also solve the PXE problem in the "Many imaging problems" thread below - though fully updating everything is the better path to take.