It depends on if you got OEM version of windows or retail and what you do with your old system. OEM is tied to motherboard so you can't move it at all.
With retail you only have license for one system, if you move it to your new build then technically you can't use it on the old system anymore so it will be without os.
I bought the OEM version so I guess that answers it...
Haha but thanks for clearing it up quick and easy for me.