There are many low-level functions within DOS that you simply can't easily get to with full windows. One of the largest needs to use a bootable DOS USB is to update drivers or a bios. It is very simple to do and will be useful for many different applications.
The first thing you will want to do is find your flash drive in your computers directory.
Mine is called PENDRIVE.
Then you will want to right click on that and hit Format
When you reach that screen make sure your FIle System is set to FAT32 and click "create an ms-dos startup disk" if available. If not then continue on to the next step.
If you did not have that option then you will want to use this tool :
Download and install that and then download the dos files from here :http://www.sevenforums.com/attachments/tutorials/42023d1260810265-ms-dos-bootable-flash-drive-create-win98boot.zip
You should then run the HP USB utility you just downloaded and check off the "Create ms-dos disk" option and then find your dos system files you just found and enter that location into the bottom data entry portion. HIt start and then you should be done. A fully Bootable DOS environment you can take wherever you want. I will use this in a later date to show you how to flash your bios. This is also especially important for computer techs updating a mass of computers manually because he or she can pop in the flash drive with the dos environment and any updates he needs to run and quickly execute them