Have you tried looking in the mainboard manual for USB boot methods/requirements? You should be able to find a PDF if you lost the hard copy, or never actually got one. If its a prebuilt, the manufacturers site may have details on a specific partition requirement for that model. Sometimes it needs to be in a specific port on very legacy equipment.
As for booting USB, if I've seen it in the BIOS menus I was always able to get it to boot off USB if the BIOS shows the actual USB device in the boot order priority. You can try unetbootin and see if you can build a unix image on another USB and get it to at least load, provided you have another USB stick to spare for a bit. The actual USB stick may not be compatible, thats not unheard of.
You can also look for a BIOS update, but you'll probably need to figure out how to install windows to apply the update anyway.
You can use win32diskimage (http://sourceforge.n...in32diskimager/) to make a 'clone' of your USB stick to a file. It'll be as big as the stick. Then you can just restore the file back to any stick and it'll work like it did before. IE if you wanna copy your windows 7 installer USB to another same or larger sized stick to try compatibility, this'll work.
