You should be able to boot from it - but I'd guess it depends on your BIOS settings (may be worth a look if it doesn't work immediately as there are settings to allow PCI cards to respond to the BIOS)
The Ultra 100 card has it's own BIOS. At a point late in the booting up process, the card will detect and list all devices attached to it. Until that happens, those devices don't really exist on your system.
If you wanted to boot from an optical drive that is hooked up to the card, and it's the only optical device on the system, setting in the system BIOS to boot from a 'non-existing' optical device may not work. And you don't have access to anything in the card's BIOS.
This is why I was unsure if it could be done. It may also depend on the card too. Maybe newer cards are better and will allow you to do more. Mine is kind of old. Of the 2 cards that my father and I have, my Ultra 133 card was the newer one...purchased in 2001...and I swapped that for his older Ultra 100 card.
The drivers for the Ultra 100 can be installed from within Windows.
The Ultra 133 card required
you to have a floppy drive to install the drivers during first boot. I had a floppy drive on the pc it was originally purchased for, but I don't have a floppy drive on this pc. On the older pc I originally had it in, that card detected and listed the devices attached to it early in the boot process, right after the listing of the drives attached directly to your system. I think that card would have better luck with booting from a device attached to it than the Ultra 100 would.
Like I said...something to experiment with some day.