How to install Windows 7 on an early 2009 Mac Pro???

I need help guys. I get a "no bootable device" during the bootcamp install process. Can you install Windows 7 on this machine with the ISO on a flash drive or is it optical drive only? I don't have USB 3.0 so, that's not it. I don't get it. Trying not to have to build a new machine here. Help!!

Surprised no one has responded.

I would think rEFIrd and W7 on DVD should work. But, hopefully you're working with W7-64bit.