Very odd. I had actually installed 10.04 on this box before. The 11.04 CD would not boot. The 9.04 CD is installing just fine. All the RAM and hard disk tests passed.
UPDATE: And yet on reboot, grub won't boot. I'll try and upgrade to 11.04 while running 9.04 from the CD.
Grub 1.5 is installed by 9.04, and Error 18 means that the BIOS is incompatible with this size of hard disk. But I had this working a few months ago, so I suspect that the version of Grub installed with 10.04 Ubuntu Linux handles this. Now I will download 10.04 and put that on a CD.