Cheers to K Mandla for posting this nugget on his blog.
I came into possession of a "new" old computer whose motherboard doesn't support booting off of the USB ports. Rather than try and burn an army CD-R discs again (which I try not to do, those miserable things) I used Plop, which K Mandla suggested a few blogs ago.
Try it now.
After you burn its 2 MB disc image to a CD-R and boot it in your drive, you'll see a very neat star screen saver with a futuristic menu to the left, offering to boot off of, among other things, USB.
This is perfect for old computers that have slow hard drives. USB drives are almost always faster than discs, and they're more reliable. Just use UNetbootin to put the distro on the USB drive and you're set to go.