first make sure that your DVD drive supports being a boot device (if you don't have the documentation, and your drive is relatively new it should...)
second, set up your BIOS so that its either first on the boot order, or right after the floppy drive.
third make sure the DVD is burned as a 'system' disc...I'm not familliar with all burning software, but I know that NERO will ask you to insert a floppy disk that's been formatted as a system disk to use for this.