Stupid answer: Yes and no.
All startup disks are boot disks but not all boot disks are startup disks. A boot disk contains enough system files to get your machine to "boot" into dos mode. A startup disk has additional files helpful in fixing or modifing your system like fdisk, format, debug, cd rom and mouse drivers...
Any bootable disk is a boot disk. You can make a boot disk buy going into format and selecting copy system files. A startup disk can be made by going to Add/remove programs and then to startup disk tab. LD