bootcamp - cannot verify digital signature

I am trying to install Win 7 Pro onto my Mac Pro (Retina late 2013) running OS X Yosemite. I have run bootcamp and created the ISO file, downloaded the software and gone as far as installing windows, but when the computer resets and starts to load WIndows I get the error message "Windows canot verify digital signature" and links to the file "applessd.sys"

what do I need to do please?