How to boot XPS 8500 from DVD/CD drive?

I have an XPS 8500 with 8.1 pre-installed and after many months of trying to accept Microsoft's misguided idea of what I want in an O/S, I finally gave up. It's the worst operating system ever created. It belongs in the dust-bin of history alongside Vista and another disastrous O/S version whose name I can't even remember. So I bought Windows 7 and want to do a CLEAN install. I want to format the drive, not install and keep 8.1.  I do not want any trace of 8.1 left anywhere on this machine. My only issue is I can't make the PC boot from the CD/DVD drive. I went to F2 setup and F12 and tried changing the boot device with the Windows 7 install disk in the drive, but it said it wasn't a bootable drive. Changing the boot sequence should in theory be very easy, but somehow Dell has made it complicated.

All I want is to tell the PC that for this start up I want to boot from the CD/DVD drive.  The manual says F2 - F12 and change it, but it doesn't say how. 

Anyone know how?