I'm running windows 7 on my PC and I need to set a VM (i.e. using virtualbox) running windows 7 (no other OS). How can i do it? I can't use my OEM licence product key to install windows 7 in the VM I guess. I want neither to buy another licence, nor to use a crack as I already have a valid licence and I need it only for testing purposes. Any hints?
I suggest you take a look at Modern.IE, where Microsoft provides free VMs (with options for Virtualbox, VmWare and other hosts depending on the host OS) with different pre-installed versions of Windows and Internet Explorer.
It is mainly aimed at web developers who want to test their sites in different versions of IE, but as far as I have tested the VMs run full Windows installations (for Windows 7 specifically, this means a Windows 7 Enterpise Edition installation).
The trial period for a VM is 90 days (meaning you should activate it with a valid license after that period) but it can be worked around e.g. with VirtualBox snapshots or simply saving the downloaded vm and reimporting it if it's an option.