iMac Early 2009 unable to install Windows through Boot Camp

[LuminousFX]

I am trying to install Windows on this iMac, early 2009, running El Capitan through boot camp.

But each time I try to do so through Boot Camp Assistant, I run into this error,

Boot Camp only supports installing Windows 7 on this Mac. please insert a USB or DVD which contains a full version of Windows 7.

I am positive I have Windows 7 disc in the superdrive, this is perplexing.

Any help is much appreciated.


Just to add on, I burnt the Windows 7 ISO via right-clicking on it and selecting "Burn"

 

[Gjwilly]

Not all Windows ISOs are bootable and if you used a Mac to burn the disc that could be part of the problem.
You will likely need to use an actual PC to burn the disc and you'll probably need to use the Windows USB/DVD Download tool.
https://www.microsoft.com/en-us/download/windows-usb-dvd-download-tool

It also needs to be an ISO for 64-bit Windows. 32-bit or 32/64-bit Hybrid discs aren't supported.