My Bios gives 3 options for Boot drives with the term 'USB' in them (Floppy, ZIP, and CD), none of which facilitate booting from a thumbdrive - I have attempted all 3.
I junked my USB floppy drive when I moved a few months ago (along with a lot of other stuff). I didn't think I would ever have a use for it again. Last time I used it, I put Memtest86+ or some similar program, on a 3.5" floppy to run during a re-boot. I ended up with a computer that would only boot into Syslinux and run Memtest86+, because the program had overwritten the bootsector on my hard drive.
IIRC at the time the PC would only boot with the floppy drive connected and that particular floppy inserted. I managed to fix that by installing a new bootloader from the Ultimate Boot CD, which I made a copy of some time ago. That bootloader still presents me with a choice every time I boot up, to boot Windows XP or boot Windows XP (I am not sure why there are 2 options, I think I only set 1 up - I was just happy to have my computer boot again and never messed with the bootloader afterwards). I would really like for a new OS to re-write the MBR and boot sector as part of it's installation, leaving me with a simple, single OS boot experience afterwards.
I have been looking into trying a different way of net installing Debian, that bmike1 linked to 2 posts up. I don't know anything about TFTS service, but I found a program OpenTFTS at Sourceforge. I have downloaded and installed it, and set the TFTS service to run as a Manual start service.
What is preventing me from attempting a net install at this point is a serious doubt on my part, that the installation can simultaneously rely on files and services on my drive on the one hand, and wipe out everything on that drive, and replace it with Linux OS on the other hand. I am not sure that I should attempt this, since it is possible that I will only succeed in making my computer un-bootable.
I have a working DVD writer, it is currently installed in my newer computer which is temporarily down for repairs (probably bad Power Supply or Motherboard, not sure). I could take it out and install it in this machine, and then buy blank disks - unless it turns out one or more of my un-labeled disks actually is blank (which I kind of doubt). I had hoped to leave the DVD drive where it is, but removing it and re-installing it into my older computer isn't out of the question. I also have an external HDD, with ~ 600 GB free space on it, if that makes things easier.