I had the following problem and would like to share my solution:

ZEN 4.01 IR7, PXE boot.

Intel D102GCC2 motherboard (rather new hardware at 08-2006)

On PXE Boot, the system freezes on "Load USB Modules"

I put the line "USB=NO" in SYS:\TFTP\CMD\z_auto.cmd en z_maint.cmd.

Then Linux does not load USB modules.

As long as you don't need USB support while imaging this works fine.
Better then disabeling USB (2.0) in the bios.

I can't test at the moment, but I think this wil also work on ZEN 6.5 /

Thomas Roes