Not to mention the fact that MS is likely to be able to detect this sort of piracy and lock you out from using the illegal product.That brings up a good point. Even if you have a legitimate copy of Windows, there are significant limitations on how and when you can install it on a new machine (real or virtual).
I have a stack of legitimate Windows install disks which can no longer be used because I have installed too many times (I tend to wipe and reinstall often as part of my work as a software developer).
Supposedly (triple underline that word in this context) there is a way to call MS and explain to them that you’re not installing on a new machine - it’s just that you’ve modified your existing machine and (again supposedly) they are supposed to be able to give you what is in effect, a variance or a waiver.
However there is theory and there is practice. This may not always work. At some point they might just say “no mas”.
If you need to scrub your machine frequently why not use ghost? It’s free on a hirens, save an image of your clean install, do your dirty work, and scrub. Faster than re-installing and doesn’t use up your licenses. I wouldn’t even try to run my test lab without ghost.