In
How do I unregister my key( the OS) so I can install it on a
different system? It may sound crazy but that's question I need
answered.
No unregistering (or unactivating, which is what you really mean)
is necessary or even possible;.
If yours is a retail version, just remove it from the old system,
install it on the new one, and activate (not register;
registration is always completely optional) it there. If it's
been over 120 days, activation should proceed normally over the
internet, just as it did the first time. If fewer than 120 days,
you'll be given an 800 number to call and activate over the
phone, but that's almost as quick and easy.
But if yours is an OEM version, be aware that the license for an
OEM version restricts it to the first computer it's installed on.
It can never legally be moved to another computer, sold, or given
away.