I am trying to install WIN7 on my newly formatted disk. I have windows on a IDE DVD connected with a USB interface cable. I cant get it to boot from DVD, why?
When you start your computer, you should be given an opportunity to enter your machine's basic "Setup"
program. As part of this setup, you should be able to tell the computer which devices to attempt to boot from, and in what order. You need to make your DVD drive the first choice there, assuming the Setup program knows that it is there as a USB-connected device.
If you're not familiar with doing that, I'd suggest you ask someone for some help that has done this sort of thing before. There can be problems that can be frustrating to deal with.
Good luck!
Access to the [u]BIOS[/u] depends on your computer (or motherboard) so check the webpage of your computer manufacturer or search BIOS with your computer's model number.
Or if you are lucky, a quick message may pop-up on the screen telling what key to hold down.
Usually, you hold-down the "Del" key, or F1 during boot (or while it's trying to boot). My newest computer (a Dell) was something different, maybe F2.
…P.S.
It's too late now, but whenever I've upgraded my operating system I buy a new hard drive and I save the old one "untouched".
If I'm "upgrading" (not doing a fresh install from scratch) I'll clone the old drive first, then pull-out the old drive and install the clone. I have a little kit like with [u]this[/u] with a USB-to-SATA adapter and a power supply so I can write to the new drive before installing it. (And, there are free cloning applications.)
While it is the case that Win 7 is the preferred version, though Microsnot proposes to trash it 2020, this always begs the question as to why you would want to install any form of Windoze on a PC? 
The only reason is of course, that you have some poorly written proprietary program that absolutely demands it. 
Mind you I am presently having problems with new installations of VLC playing DVDs!