Really really rough one here.

How specific does this get?
I'm trying to install Red Hat onto a Compaq Proliant 3000 server machine. Before the installation is started, both Pentium II 300mhz processors work dandy. When it's done installing only 1 processor works. Red Hat can boot to both the SMP and single processor kernels that it installs, though on the SMP boot it said something about not being able to use the second processor. This is no big surprise since the computer itself was saying that it couldn't deal with the processor for whatever reason.
Well that's no problem right? We'll just fix the problem with the processor, done, and now we'll boot to Red Hat. Except this time it doesn't even get as far as Grub before it says "Invalid Boot Disk." Anyone encountered anything like this before? I highly doubt it's a problem with Red Hat, however much I would like to blame the problem on Red Hat.

I'm only qualified to suggest the obvious - make sure there isn't a floppy or CD in a drive when you boot. Take a look at the drive boot sequence in the BIOS and make sure everything's okay (usually you want it to try the HDD first anyway.)
Going out on a limb - take a look at grub.conf and where it's looking for the kernel images...Still you wouldn't expect it to complain until it actually tried to boot from a bad location.
I think RedHat also does some weird stuff with a temporary RAM disk on boot that I don't understand...