Pulled from the Boot Camp Assistant resource files, these are the only ones that can boot from a USB flash drive into Windows:
Code:
<string>IM130</string>
<string>MM50</string>
<string>MP60</string>
<string>MB80</string>
<string>MBP90</string>
<string>MBA40</string>
So, unless you have a 2012 iMac, you cannot boot it from USB into Windows.
Can you post a screenshot of what you see on that media?