You can copy data from your existing hard drive but that's all. You can't carry Windows over to your new build, it won't work. Moreover, if it's a factory-built machine which came with Windows pre-installed, that copy of Windows is permanently linked to that computer and can't be transferred to another one. Those are the terms of Microsoft's licencing agreement.
On your new build you will have to install Windows from scratch so that Windows Setup can create a unique, hardware-specific Product ID which is used for Product Activation.