OK to add:
it looks like it is automatically selecting the PXE boot environment (as it shows the "Altiris Auto PXE management ...." that shows up after it automatically selects the WinPE image).
But once it does that I get a:
PXE02: Exiting broadcomm PXE boot
and then the job fails.
This has never happened before, and I know it works on other PCs as I can push the image to a different PC and it would work fine.
I am going ot try another reboot in the meantime (once the WinPE image finishes rebuilding; my only theory so far).
Chris