What exactly happens when activating the same windows key on two installations

I have a Windows 7 HP Key and was wondering how I would go about reinstalling my machine. The obvious way would be to just backup all data, format the drive, and install Windows anew.

I explicitly do not want to go this way, because I want to ensure that all my programs are running how I need them BEFORE formatting the hard drive. So, obviously, I need to run two installations at the same time.

Considering that I don't know how long it will take for me to get all programs running on the new installation, I probably will need to activate the key on my second installation.

That raises the question in the title; What exactly will happen when I do this? I can think of three possible outcomes

• The old windows installation will deactivate, because a new one is activated
  
  
  
  
  • This makes the most sense from a corporative POV I guess, so you can be sure that the same key is always only used on one installation only
  
  


• The new windows installation can not be activated
  
  
  
  
  • This would bring a follow-up question: how do you even deactivate a windows key, so you could use it on a new installation?
  
  
  


• Nothing happens(tm): I can use both installations as usual with the same key activated
  
  
  
  
  • This would seem strange, because: What's the point of activating it if it isn't somehow tracked on the MS servers?
  
  

Answer

When an activation key is invalid, the activation simply fails. You can finish installing Windows, but the resulting installation will lock down in 30 days. More details here.

MS often resets activation keys when people give them a good reason for doing so. I'd guess your reason rates, since you're not trying to use the same key on two machines.

You don't make it clear exactly what you mean by "run two installations". Will you be swapping hard drives? Or will you be using two partitions on the same drive? If the latter, you'll end up with a Windows installation on a non-C drive. That will work, but it can be a nuisance. I speak from personal experience.

An alternative approach would be to make a disk image before installing the new OS. Then if you decide to roll things back, you just restore the old image.