What you are trying achieve is creating a custom image.
You don't have to use Microsoft Deployment Toolkit for this purpose. Thing is, you could do this with MDT easily if you knew how to use it. Otherwise, it is quite confusing at the beginning. (With all those task sequences and stuff.) I found that creating custom images manually gives you more understanding about the imaging process.
With that said, there are guides on the internet about how to do this. This one is a really good example. One thing that is not mentioned in this guide that if you want a fully unattended installation, you would want to skip computer name dialog. To do this, use an asterisk for computer name in unattend.xml.
Please note that some 3rd party programs such as antiviruses (in my case it was NOD32) may prevent sysprep process and fail when you are deploying the OS.
After you are done sysprepping and machine is shut down, boot with Windows installation media and once you are at setup screen, press Shift + F10 to open up command line. Here you can use dism to capture an image. For example:
Dism /Capture-Image /ImageFile:E:\my-windows-partition.wim /CaptureDir:C:\ /Name:"My Windows partition"
Where E: could be an external HDD or something else, while C: being Windows installed partition. (If you don't have an external HDD you can simply create this file in C: drive too.)
Then you will replace that my-windows-partition.wim with install.wim in Windows 8.1 media.
To do that, copy contents of an 8.1 ISO to a folder. Replace install.wim in sources folder with your customized my-windows-partition.wim It has to be named as install.wim though.
Then you can use a 3rd party tool to create an ISO from that content.
While all these things may require quite some effort to do, you will achieve exactly what you want.
By the way, for having an easier and more fluent process, you should use virtual machines if you are able to.
I would like to explain all the steps that you are going to do but this is impossible due to how this subject is really extensive. So here are some references:
How to Create a WinPE ISO
How to Capture an Image
How to Create a Bootable ISO From Files and Folders
Also: How to Bypass Metro Start Screen
Edit Here:
I felt this post was not so explanatory to beginner users. So, i will explain like this:
Let's say you are replacing your computer. But you want to transfer everything exactly in its current state. Can you do that? Well, yes. You would sysprep your computer so your OS would be hardware free. Meaning you could install your OS on other computers. But how can you transfer it? Well, you could make images of your partitions. Let's say, whole C: drive to a single wim file. Then you could restore everything, using this wim file, on another computer. So it will work just like how it was working on the old computer. Cool, right? Yeah, but, what if i said that you could replace that wim file with the one in Windows 8.1 DVD? What would happen? You could install Windows with that image file to every computer, lol. Yes, you can do that. Basically, Windows installation uses a base image file in Windows 8.1 DVD to install Windows. What you have done here is, changing that wim file with your customized one.
So, i tried to explain the logic behind windows installation to clarify my answer. There are more things to consider about wim files, sysprep and windows installation but, i don't want to make this post any longer, cause it is already very long.
