This would still be illegal becuase the other user is still using the operating system. You installing it on your drive would mean there are 2 of the same copies.
I not sure whether you will be able to activate it becuase windows produces a Computer ID number and according to your configuration, the Number won't match the one the other user registered.
whenever you activate, you give Microsoft the id number. If you register now, Microsft will see two different ID (2 different computers) and won't give you an activation number.
I'm not too sure about this, I'm just saying what I think. Hopefully someone can confirm this.