I've flashed the BIOS on a number of Dell desktops, always with a bootable
diskette or CD. I don't think I would trust a Windows flash process.

Unfortunately, it seems that Dell doesn't provide a .ISO file for your 1721.

I have not faced exactly that situation, myself. But, if I were in your
position, I would copy Dell's A07_1721.EXE file to a blank CD. Then I'd boot
from a DOS CD (I've got a few), switch CDs, and launch A07_1721.EXE. (Once DOS
is in memory, you don't need the OS CD in order to launch a program.)

If you don't have a bootable DOS CD, one of the DOS environments from the
"ultimate boot CD" collection might do the trick. Get the UBCD from here:

http://www.ultimatebootcd.com/index.html