Cannot PXE install an XP SP2 RIS image, SP1 image fine

I have a perfectly working SP1 RIS image and when I take this image,
put it on a machine, install SP2 and then make a new image of this
onto the RIS server, I cannot install this SP2 image onto a client PC
- the PXE setup crashes just after "Waiting for Windows to Start" with
a blue-screen STOP 0x000000BB.

This happens every time - I have made the image twice, just to check.
The XP SP1 image works fine. I know about swapping the NTOSKRNL.EXE
versions in the flat-file image on the RIS server, it's not that.

My suspicion is that the PXE boot rom on the 3com 3C905C-TX cards
needs updating? Why would it work with an SP1 image but not an SP2
image?
Any ideas much appreciated - due the the stupid inability to disable
the SP2 firewall as an install option, I have to travel miles in order
to get an SP2 PC working so I can then configure the group policy to
stop the damn firewall preventing me RDPing in on all the other
machines I have to upgrade to SP2 ...

PK