thank you for your replies.
Well, I can't boot into Xen at all.
It is the very first time attempting to boot into Xen, before setting up any domUs. Right at the bottom of page 3 of the tutorial, where we shutdown for reboot.
As I said I have used the tutorial to install Xen on other PCs with no problems, so it seems to be a problem specific to this machine. (Sunfire X2100, Opteron singe core, 80GB SATA, 2GB RAM)
I read on the Xen Users list that it tries in desperation to boot from NFS if the proper root fails, hence I wondered if it was an issue with the 64 vs 32 bit processor. I'm pretty sure the grub is pointing it to the right place.
So any other thoughts?