When I boot the xubuntu installation CD, the screen with language selection loads, I select english and then go to "install CD". After hitting install, I get the message that "Resource is not an IRQ entry". I get the xubuntu logo in blue and a bar moving back and forth. Then it immediately goes to something called BusyBox v1.1.3 and a command line that begins (initramfs) and the a blinking underscore.

Does anyone know what's going? Any suggestions on how to get this working?

It's an old Compaq Presario 1500T (Pentium 4) with what should be a completely clean hard drive (I ran Boot and Nuke on it).

No one on the ubuntu forum could help, but obviously it's a big problem, so I was hoping someone here might know.

Hmm, that appears to be a Compaq and IBM specific bug.
Is it a livecd? Does it work if you use it as such?
There is a comment about thre error in the link below but it was caused by a non-existent floppy drive - I don't know whether that is your problem :
http://odysseus.wordpress.com/2007/1...nd-ubuntu-710/

It is a live cd, but it doesn't work as a live cd. A similar error message comes up when I try booting it that way, and then it kicks me over to BusyBox just as if I had tried to install.

That guy got the desktop up. I can't even get that far, so I don't know if it would show a non-existent floppy disk drive or not. The computer I'm trying to install on is actually old enough that it really does *have* a floppy disk drive as well as a CD-ROM drive.

Have you tried changing BIOS settings? Disabling - or even removing - one or more components may help as it appears to be a matter of IRQs. Unfortunately, there is no knowing which one could be the culprit so you'll have to experiment a bit.