I second using WAIK. It is alot easier than creating an Unattend file by hand. This tutorial helped me out quite a bit: [url]http://theitbros.com/sysprep-a-windows-7-machine-–-start-to-finish/[/url] .
As for activation, that is a tricky issue. My institution uses a KMS server. Somewhere I read that newer machines have the activation key hardwired into the computer somewhere. However that may only be for UEFI installations. If your computers do not have an activation code on the outside and they came with Windows pre-loaded then they key might be in the machine. That being the case any valid copy of Windows is supposed to work without having to enter a product key.
So if my thinking is correct you should be able to simply image the computer and Windows will pull the product key automatically from where ever it is stored. My laptop worked like that. I reinstalled Windows and it never asked me for a product key, but still worked. However I am not sure how that would affect you Unattend file. The only other solution is to create a separate Unattend file for each computer with the product key for that machine which is not an ideal solution.