Heres what you need to know:
1. When you boot from a usb key fob, in most case it will emulate ether a fdd or a hdd, you need to know which one your BIOS users.
because it not a simple case of load the first scetor and jump to it.
If you use a floppy image for a bios that emulates hdd, it will not boot, its as simple as that.
2. To see what your BIOS user's, put any usb fob in your PC and reboot, then enter bios and go to boot order see if, it says usb fdd or usb hdd.
If it does not say ether fdd or hdd and just usb stick or something like that its most likely to be hdd.
3. Now to boot as a floppy its as simple as geting a bootable floppy image and puting it on the usb device with a program like dd for windows or dd in linux.
For Hdd you most first format it fat16 (unless it a big fob) and than make it bootable and add your boot sector, see my tut here:
http://www.dex4u.com/USBboot.htmNote: on later vers of HxD you can not use delete, but you use replace instead, so if you follow that tut and delete does not work use the replace option.
4. Last but not least, you are much better of to start with, using dos to load your OS by making it in a format that Dos can load eg: mz exe.
Then all you need to do is format it and than click make bootable, the work under win98, but under xp you need to use the HPformat and maybe freedos files.
I have written a windows program to do this for you but, have not got round to finishing it.