There are two bootloaders that you might try also.
On older machines or machines with "quirky" bios, booting from the cd-rom drive works where other methods fail.
You might look into downloading plop or riplinux.
Either one may be your solution when the computer's bios are at fault.
Both can be burned to cds and used in your cd-rom drive when you start your computer.
Once they auto-run you can select drive to boot from or usb to boot or partition to boot on a given drive. (plus more in the case of rip linux)
Grub4Dos is part of Rip Linux and it works very well indeed.
Another thing worth mentioning, making sure that the drive you use has a valid, undamaged 'Dos label' and that the drive has it's "boot flag" active and has a valid, error-free file system structure on it.
Hope that info is helpful.