I have a hard drive that im going to use for my build

[acer2dn]

I have windows installed on it, If i build a new pc and use the same drive, will i need to install windows 10 again or no

 

[Dugimodo]

It may work as is but will need reactivating depending on the license type. But it's always better to start over with a new machine, you are asking for trouble otherwise.

So I would say yes, you should reinstall windows.
Having said that I have successfully swapped in a working copy of windows to a different machine, booted up and changed the key and activated without issue. I just can't guarantee that will be your experience.

 

[USAFRet]

I have windows installed on it, If i build a new pc and use the same drive, will i need to install windows 10 again or no

Yes, you will probably need to reinstall the OS.

Also, for the Win 10 licensing...
Read and do this before you change any parts:
https://support.microsoft.com/en-us/help/20530/windows-10-reactivating-after-hardware-change
http://www.tomshardware.com/forum/id-3164428/windows-build-1607-activation.html