Hey there, cztrollolcz.
Basically another drive (as @helpstar has suggested) is your best option. From what I understand you plan on using this drive with the new build, so if this is the case the Windows installation won't be needed, because your new computer won't be able to boot to Windows with that drive. This is because the hardware is different and you can't use an installation of Windows from one computer to another (except in some rare cases where the motherboards are the same). So having this in mind you could either get an external drive in order to store all of your data to, so that you can make a fresh install of Windows to your current HDD and reformat it. Or you can get a second internal HDD, install the new OS on it and use your old HDD as a secondary storage drive, as @helpstar has suggested.
Hope that helps.
Boogieman_WD